OMAP: McBSP: Fix CLKR and FSR signal muxing

Fix bit clear. Now it clears all other bits than mask bit where it should
clear only it.

Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Acked-by: Paul Walmsley <paul@pwsan.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>