ASoC: davinci-mcasp: Active slots depend on active serializers

Active slots count depends on the number of channels in the stream
and the number of active serializers. Each serializer will handle
at most the number of channels specified via 'tdm-slots' parameter
in DT.

There are two possible scenarios:

- Single serializer: channel count fits in the max slots supported by
  McASP serializers, active slots is same as channel count

- Multiple serializers: channel count is bigger than max slots supported
  by a serializer. Channel count determines how many serializers are
  needed at their max slot count configuration

Signed-off-by: Misael Lopez Cruz <misael.lopez@ti.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>