ASoC: omap-mcpdm: Fix threshold and dma configuration

DMA packet_size must be configured based on the McPDM FIFO threshold
value, number of channels.
Due to the FIFO operation the DMA muse be configured differently for
playback, and capture.
At the same time fix the McPDM threshold values used for playback, and
capture to avoid broken code.

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