omap: McBSP: Use cache when modifying individual register bits

omap: McBSP: Use cache when modifying individual register bits

Change the way McBSP registers are updated: use cached values instead of
relying upon those read back from the device.

With this patch, I have finally managed to get rid of all random
playback/recording hangups on my OMAP1510 based Amstrad Delta hardware. Before
that, values read back from McBSP registers to be used for updating them
happened to be errornous.

From the hardware side, the issue appeared to be caused by a relatively high
power requirements of an external USB adapter connected to the board's printer
dedicated USB port.

I think there is one important point that makes this patch worth of applying,
apart from my hardware quality. With the current code, if it ever happens to
any machine, no matter if OMAP1510 or newer, to read incorrect value from a
McBSP register, this wrong value will get written back without any checking.
That can lead to hardware damage if, for example, an input pin is turned into
output as a result.

Applies on top of patch 3 from this series:
[PATCH v9 3/4] OMAP: McBSP: Introduce caching in register write operations

Tested on OMAP1510 based Amstrad Delta using linux-omap for-next, commit
fb7380d70e041e4b3892f6b19dff7efb609d15a4 (2.6.33-rc3+ dated 2010-01-11).
Compile-tested with omap_3430sdp_defconfig.

Signed-off-by: Janusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Acked-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Acked-by: Jarkko Nikula <jhnikula@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>