ASoC: omap: mcbsp: Use uniform st_data pointer initialization

In this way we can save few lines, and have uniform way of initializing the
st_data in all functions.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
Signed-off-by: Liam Girdwood <lrg@ti.com>