spi: lantiq-ssc: Fix warning by using WQ_MEM_RECLAIM

spi: lantiq-ssc: Fix warning by using WQ_MEM_RECLAIM

The lantiq-ssc driver uses internally an own workqueue to wait till the
data is not only written out of the FIFO but really written to the wire.
This workqueue is flushed while the SPI subsystem is working in some
other system workqueue.

The system workqueue is marked as WQ_MEM_RECLAIM, but the workqueue in
the lantiq-ssc driver does not use WQ_MEM_RECLAIM for now. Add this flag
too to prevent this warning.

This fixes the following warning:
[    2.975956] WARNING: CPU: 1 PID: 17 at kernel/workqueue.c:2614 check_flush_dependency+0x168/0x184
[    2.984752] workqueue: WQ_MEM_RECLAIM kblockd:blk_mq_run_work_fn is flushing !WQ_MEM_RECLAIM 1e100800.spi:0x0

Fixes: 891b7c5fbf61 ("mtd_blkdevs: convert to blk-mq")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Link: https://lore.kernel.org/r/20200717215648.20522-1-hauke@hauke-m.de
Signed-off-by: Mark Brown <broonie@kernel.org>