media: staging: bcm2835-camera: Provide lock for vb2_queue

Use the device mutex to protect the vb2_queue.
This allows to replace the ad-hoc wait_{prepare, finish}
with vb2_ops_wait_{prepare, finish}.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>