ASoC: omap: mcbsp: Remove redundant checks for the st_data pointer

The parent functions of omap_st_start/stop also checks the validity of the
st_data pointer so we do not need to do it again inside of omap_st_start/stop

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>