It's quite cricial to clear error flags because SAI might hang if getting
FIFO underrun during playback (I haven't confirmed the same issue on Rx
overflow though).

So this patch enables those irq and adds isr() to clear the flags so as to
keep playback entirely safe.
Signed-off-by: NNicolin Chen <Guangyu.Chen@freescale.com>
Signed-off-by: NMark Brown <broonie@linaro.org>