ASoC: omap-mcbsp: Simplify the mcbsp_start/_stop function parameters

We either start/stop TX or RX, never both. Move the tx/rx direction
selection within the functions.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
Tested-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
Signed-off-by: Mark Brown <broonie@kernel.org>