[media] marvell-cam: no need to initialize the DMA buffers

This was an old debugging thing from years ago.  It's only done at
initialization time, but it's still unnecessary; take it out.

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