ASoC: davinci-mcasp: Fix XDATA check in mcasp_start_tx

The condition for checking for XDAT being cleared was not correct.

Fixes: 36bcecd0 ("ASoC: davinci-mcasp: Correct TX start sequence")
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org