staging: bcm2835-camera: Remove bulk_mutex as it is not required

There is no requirement to serialise bulk transfers as that is all
done in VCHI, and if a second MMAL_MSG_TYPE_BUFFER_TO_HOST happened
before the VCHI_CALLBACK_BULK_RECEIVED, then the service_callback
thread is deadlocked.

Remove the bulk_mutex so that multiple receives can be scheduled at a
time.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>