[media] marvell-cam: Don't signal multiple frame completions in scatter/gather mode

There is only one frame known to the DMA engine in scatter/gather mode, but
it still tells us that any or all of frames 1-3 are done at each completion
interrupt.  Avoid the creation of junk frames by being sure to only
"complete" one on each interrupt.

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