[media] marvell-cam: Increase the DMA shutdown timeout

Experience shows that, on the Armada platform, it can take as much as 120ms
for the DMA engine to actually shut down after it has been told to.  So a
40ms timeout is not adequate; use 150ms instead.  Also make sure we don't
leave the DMA_ACTIVE flag set once things are down.

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