ASoC: fsl_sai: Separately enable interrupts for Tx and Rx streams

We only enable one side interrupt for each stream since over/underrun
on the opposite stream would be resulted from what we previously did,
enabling TERE but remaining FRDE disabled, even though the xrun on the
opposite direction will not break the current stream.

Signed-off-by: Nicolin Chen <Guangyu.Chen@freescale.com>
Acked-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>