ASoC: fsl_sai: Reduce race condition during TE/RE enabling

For trigger start, we don't need to check if it's the first time to
enable TE/RE or second time. It doesn't hurt to enable them any way,
which in the meantime can reduce race condition for TE/RE enabling.

For trigger stop, we will definitely clear FRDE of current direction.
Thus the driver only needs to read the opposite one's.

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