Asoc: msm: compr drain/flush fixes

Add missing synchronization between handling of flush and
drain in the compressed audio driver.

Change-Id: I01b6d2c4fb372be483022d1c3430b9369dea34f7
Signed-off-by: Haynes Mathew George <hgeorge@codeaurora.org>
Signed-off-by: Dhananjay Kumar <dhakumar@codeaurora.org>