ASoC: fsl_sai: Add disable operation for the corresponding data channel.

Enables/Disables the corresponding data channel for tx/rx operation.
A channel must be enabled before its FIFO is accessed, and then disable
it when tx/rx is stopped or idle.

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