ASoC: davinci-mcasp: Correct RX start sequence

Follow the sequence described in the TRMs when starting RX.
Write to RXBUF register was not correct and there is no need to release the
RX state machine/Receive frame sync generator twice.

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