media: ti: cal: Implement get_frame_desc for camera-rx

CAL uses get_frame_desc to get the VC and DT for the incoming CSI-2
stream, but does it in a bit hacky way. Clean this up by implementing
.get_frame_desc to camera-rx, and calling that from cal.c.

No functional change intended.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>