ARM: OMAP: Fixes to omap_mcbsp_request function

Bootloader may let McBSP logic running so make sure that block is idle
before requesting IRQs. Also make sure that TX and RX waitqueues are
initialized before request_irq.

Signed-off-by: Jarkko Nikula <jarkko.nikula@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>