msm: camera: Unlock mutex in error case in ISP driver

In ISP v4l2 subdev close handler, if open count is zero,
error is returned without unlocking mutex. Unlock mutex and return
error.

Change-Id: I3c0e4fb34609734de343d605d0f7c773393d0763
Signed-off-by: Sreesudhan Ramakrish Ramkumar <srramku@codeaurora.org>