ASoC: davinci-mcasp: Correct TX start sequence

Follow the sequence described in the TRMs when starting TX. This sequence
will make sure that we are not facing with initial channel swap caused by
no data available in McASP for transmit.

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