msm: ais: Synchronize v4l2 subscribe and unsubscribe event

Serializing msm_subscribe_event and msm_unsubscribe_event to
prevent possibility of use-after-free if same event is
unsubcribed before v4l2 subscribe and unsubscribe event.

Change-Id: Ia5ba7aa1338982b3a99616a2981e415c28f8b4a6
Signed-off-by: Rahul Sharma <sharah@codeaurora.org>