ASoC: omap-mcbsp: Code cleanup in omap_mcbsp_dai_hw_params

To make the code a bit more readable, change the indexed
references to the omap_mcbsp_dai_dma_params elements with
pointer.

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>