msm: camera: Restore spin lock in error case

During get_buf(), spin lock is not restored in error case. Restore
it to avoid watch dog bark.

Change-Id: I80d1be1c3e09c184816b66f8bd07f38b2f5f3663
Signed-off-by: Sreesudhan Ramakrish Ramkumar <srramku@codeaurora.org>