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: Ib5fcb6f683b8c997121b3f4fb301eb34b76c2c1b
Signed-off-by: Trishansh Bhardwaj <tbhardwa@codeaurora.org>