ASoC: fsl_sai: Improve fsl_sai_isr()

This patch improves fsl_sai_isr() in these ways:
1, Add comment for mask fetching code.
2, Return IRQ_NONE if the IRQ is not for the device.
3, Use regmap_write() instead of regmap_update_bits().

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