media: v4l: subdev: Clear frame descriptor before get_frame_desc

Clear frame descriptor before calling transmitter's get_frame_desc() op.
Also remove the corresponding memset() calls from drivers.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>