[media] staging: media: omap4iss: video: Don't WARN() on unknown pixel formats

When mapping from a V4L2 pixel format to a media bus format in the
VIDIOC_TRY_FMT and VIDIOC_S_FMT handlers, the requested format may be
unsupported by the driver. Return a hardcoded default format instead of
WARN()ing in that case.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>