ASoC: davinic-mcasp: Adopt the AFIFO/DMA configuration to the stream (dynamic depth)

Configure the AFIFO numevt parameter based on the requested tx/rx_numevt,
active serializers and period size in words.
In this way McASP can adopt it's (and the DMA) configuration runtime and
can pick the most optimal setup which satisfy the parameters.
This way we do not need to place any constraint on the stream itself,
allowing application greater freedom on how they want to set up ALSA.

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