msm: camera: flash: Remove race condition in subdev close

Subdev close is calling flash_shutdown() to flush req and release
device operation. flash_subdev_close() does the mutex lock already.
Removing same mutex lock opertion from flash_shutdown() function
to remove race condition.

Change-Id: Id2fb7c1b73d4294d1e164df3b44e51455ba15b7f
Signed-off-by: Jigarkumar Zala <jzala@codeaurora.org>