msm: camera: Correct the return logic for error case

Currently for the flush request error case, mutex unlock is
executed without mutex lock which result in instability of
system. This change returns with appropriate error code
if error occurs.

Change-Id: I2ff0056002c7d96a7100295f2ed6ba5cc0da2254
Signed-off-by: Jigarkumar Zala <jzala@codeaurora.org>