[media] marvell-cam: ensure that the camera stops when requested

The controller stop/restart logic could possibly restart DMA after the
driver things things have stopped, with suitably ugly results.  Make sure
that we only restart the hardware if we're supposed to be streaming.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>