ASoC: msm: Set the pending buffer flag on driver underrun.

-This is to handle a scenario wherein a pause command is issued
 after an underrun. On resume the driver checks if pending buffer
 is set,if not then we dont write any buffer to DSP. This results
 in an inifinte wait in HAL.

Change-Id: Id89b83e9a11b8561f845ec3f04bf5e42452ca089
Signed-off-by: Ashish Jain <ashishj@codeaurora.org>