ASoC: fsl_sai: Reset FIFOs after disabling TE/RE

SAI will not clear their FIFOs after disabling TE/RE. Therfore, the driver
should take care the task so as not to let useless data remain in the FIFO.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>