ASoC: davinci-mcasp: When stopping TX/RX stop the AFIFO as the last step

The AFIFO should not be stopped (or started for that matter) when McASP is
running since it can cause unpredictable issues because we are switching off
AFIFO for the direction which was handling the requests from McASP and was
generating DMA request toward the system DMA.

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