omap: McBSP: Use macros for all register read/write operations

There are several places where readw()/writew() functions are used instead of
OMAP_MCBSP_READ()/WRITE() macros for manipulating McBSP registers. Replace
them with macros to ensure consistent behaviour after caching is introduced.

Tested on OMAP1510 based Amstrad Delta.
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>