msm: camera: Add support for not accepting pkts after streamoff

There can be a race condition where unlink happens and user
driver can send the packets to kernel. Avoid this condition
by validating link handle.

Change-Id: I13e53b98aab309ed78ac6e44d4104b764f8014ea
Signed-off-by: Viswanadha Raju Thotakura <viswanad@codeaurora.org>
Signed-off-by: Karthik Anantha Ram <kartanan@codeaurora.org>