ASoC: omap-mcbsp: Remove unused FRAME dma_op_mode

The frame dma_op_mode has never been used, and it is just creating
confusion for users/developers.

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>