ASoC: davinci-mcasp: Clear TXSTAT register before activating serializers

Follow the guideline from the TRM:
Before starting, clear the respective transmitter and receiver status
registers

To avoid stale state stored in the status registers.

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