msm: camera: Resetting last flush req to zero in streamoff

last flush request needs to be set to zero at the time of streamoff, as
streamon will start with request id 1.

Change-Id: I17a868dc048a6777bd1e5faa526cff61efdfc3ea
Signed-off-by: Jigarkumar Zala <jzala@codeaurora.org>