mailbox: pcc: Fix crash when request PCC channel 0

mailbox: pcc: Fix crash when request PCC channel 0

When PCCT is not available, kernel crashes as below when requests PCC
channel 0. This patch fixes this issue.

[    0.920454] PCCT header not found.
...
[    8.031309] Unable to handle kernel NULL pointer dereference at virtual address 00000010
[    8.031310] [0000000000000010] user address but active_mm is swapper
[    8.031312] Internal error: Oops: 96000004 [#1] PREEMPT SMP
[    8.031313] Modules linked in:
[    8.031316] CPU: 31 PID: 1 Comm: swapper/0 Tainted: G        W       4.13.0-rc1 #18
[    8.031317] Hardware name: AppliedMicro(R) 07/20/2017
[    8.031318] task: ffff809ef3b08000 task.stack: ffff809ef3b10000
[    8.031322] PC is at pcc_mbox_request_channel+0x8c/0x160
[    8.031325] LR is at xgene_slimpro_i2c_probe+0x1c0/0x378
[    8.031326] pc : [<ffff000008899450>] lr : [<ffff000008819dac>] pstate: 00000045
[    8.031327] sp : ffff809ef3b13bd0
[    8.031327] x29: ffff809ef3b13bd0 x28: ffff000008ed90a0
[    8.031329] x27: ffff000009091000 x26: ffff000008e50470
[    8.031330] x25: ffff000008ed9100 x24: ffff809eefd9ac30
[    8.031332] x23: 0000000000000000 x22: ffff0000090e3e10
[    8.031333] x21: ffff0000090e3000 x20: 0000000000000000
[    8.031335] x19: 0000000000000000 x18: 0000000000087ffc
[    8.031336] x17: 2fe48d76a78303f0 x16: 0000000000087ffc
[    8.031337] x15: ffff000000000000 x14: 0000000000000000
[    8.031339] x13: 0000000000000000 x12: 0000000000000018
[    8.031340] x11: 0000000000000018 x10: 0101010101010101
[    8.031342] x9 : 0000000000000000 x8 : 7f7f7f7f7f7f7f7f
[    8.031343] x7 : fefefefeff6b646d x6 : 0000008080808080
[    8.031345] x5 : 0000000000000000 x4 : 0000000000000001
[    8.031346] x3 : 0000000000000000 x2 : ffff000008819b64
[    8.031348] x1 : 0000000000000000 x0 : 0000000000000000
...
[    8.031393] Call trace:
[    8.031394] Exception stack(0xffff809ef3b13a00 to 0xffff809ef3b13b30)
[    8.031395] 3a00: 0000000000000000 0001000000000000 ffff809ef3b13bd0 ffff000008899450
[    8.031397] 3a20: ffff809f7e1f9a10 ffff000008f60be0 0000000000000001 ffff809ef3b13b7c
[    8.031398] 3a40: ffff809f7e1f9a10 0000000000000000 ffff000009091000 0000000000000003
[    8.031399] 3a60: ffff000009091000 0000000000000003 ffff809ef3b13a80 ffff0000084e0794
[    8.031400] 3a80: ffff809ef3b13a90 ffff00000850bb64 ffff809ef3b13ad0 ffff00000850bf34
[    8.031402] 3aa0: 0000000000000000 0000000000000000 ffff000008819b64 0000000000000000
[    8.031403] 3ac0: 0000000000000001 0000000000000000 0000008080808080 fefefefeff6b646d
[    8.031404] 3ae0: 7f7f7f7f7f7f7f7f 0000000000000000 0101010101010101 0000000000000018
[    8.031405] 3b00: 0000000000000018 0000000000000000 0000000000000000 ffff000000000000
[    8.031406] 3b20: 0000000000087ffc 2fe48d76a78303f0
[    8.031409] [<ffff000008899450>] pcc_mbox_request_channel+0x8c/0x160
[    8.031410] [<ffff000008819dac>] xgene_slimpro_i2c_probe+0x1c0/0x378
[    8.031413] [<ffff0000085e84dc>] platform_drv_probe+0x50/0xbc
[    8.031414] [<ffff0000085e68a4>] driver_probe_device+0x21c/0x2d0
[    8.031416] [<ffff0000085e6a04>] __driver_attach+0xac/0xb0
[    8.031417] [<ffff0000085e4a78>] bus_for_each_dev+0x58/0x98
[    8.031418] [<ffff0000085e61e4>] driver_attach+0x20/0x28
[    8.031419] [<ffff0000085e5e0c>] bus_add_driver+0x1c8/0x22c
[    8.031421] [<ffff0000085e7324>] driver_register+0x60/0xf4
[    8.031422] [<ffff0000085e8420>] __platform_driver_register+0x4c/0x54
[    8.031425] [<ffff000008e96dd0>] xgene_slimpro_i2c_driver_init+0x18/0x20
[    8.031426] [<ffff000008083144>] do_one_initcall+0x38/0x124
[    8.031429] [<ffff000008e50d0c>] kernel_init_freeable+0x190/0x22c
[    8.031431] [<ffff0000089eac30>] kernel_init+0x10/0xfc
[    8.031432] [<ffff000008082ec0>] ret_from_fork+0x10/0x50
[    8.031434] Code: cb030e63 8b030013 b140067f 54fffda8 (f9400a61)
[    8.031448] ---[ end trace 14eb48a4e1e1f9fb ]---

Signed-off-by: Hoan Tran <hotran@apm.com>
Acked-by: Prashanth Prakash <pprakash@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>