[media] marvell-cam: fix the green screen of death

I had learned through hard experience that dinking around with the DMA
descriptors while the C1_DESC_ENA enable bit was set is a recipe for all
kinds of truly malicious behavior on the hardware's part, regardless of
whether the DMA engine is actually operating at the time.  That
notwithstanding, the driver did so dink, resulting in "green frame"
captures and the death of the system in random, spectacular ways.

Move the tweaking of C1_DESC_ENA to the same function that sets the
descriptor so we know that we'll never try to set a descriptor while that
bit is set.

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