ASoC: davinci-mcasp: Channel count constraints for multi-serializer case

Set channel count constraints for multiple serializers case. On McASP
the active channels mask is the same for all the serializers. With the
current implementation this means that if more than one serializers is
used, all TDM slots have to be active on all serializers. The patch
sets the channel count constraints according to number of RX and TX
serializers.

Reported-by: Misael Lopez Cruz <misael.lopez@ti.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>