ASoC: Samsung: I2S: Flush FIFO after stop

Flush the FIFO while stopping the channel rather than starting.
This saves time during stream start and keeps the FIFOs clean
when the channel is idling.

Signed-off-by: Jassi Brar <jassi.brar@samsung.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>