ASoC: omap-mcbsp: Restructure the code within omap_mcbsp_dai_hw_params

In preparation for the extended threshold mode (sDMA packet mode
support), the code need to be restructured.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Acked-by: Jarkko Nikula <jhnikula@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>