staging: bcm2835/mmal-vchiq: unlock on error in buffer_from_host()

We should unlock before returning on this error path.

Fixes: 7b3ad5ab ("staging: Import the BCM2835 MMAL-based V4L2 camera driver.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>