ASoC: davinci-mcasp: Implement configurable dismod handling

If the dismod is specified in the DT node, use the specified custom value
to configure the drive on state of the inactive TX slots.

If the dismod is not present or booted in legacy mode, the dismod is set
to low as it was the original behavior.

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