msm: camera: isp: use a spin_lock instead of a rwlock

The only entity that uses the read side of the rw_lock
is the interrupt handler so there isn't any benefit to
multiple readers running at the same time.  Convert
the rwlock to a spinlock to simplify the semantics.

Change-Id: Ic0dedbade4cf39f42e70f69d1cd9c682495ff929
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Soundrapandian Jeyaprakash <jsoundra@codeaurora.org>