ASoC: omap-mcbsp: Use DMA packet mode for non mono streams on OMAP3+

Take the DMA packet mode into use when the McBSP is configured in element
dma_op_mode if the stream is not mono.
In this way we transfer one sample from/to McBSP FIFO upon DMA request.
This change only affects OMAP3+ versions, where the McBSP ports have FIFO.

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>