ASoC: omap-mcpdm: Set 24msbits constraint

McPDM internal FIFO is 24 bit wide. From the 32 bit sample
8 bit is discarded. Let application know about this
via msbits constraint.

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