[media] omap3isp: 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 format instead of
WARN()ing in that case.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>