media: v4l: subdev: Add trivial set_routing support

Add trivial S_ROUTING IOCTL support for drivers where routing is static.
Essentially this means returning the same information G_ROUTING call would
have done.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Co-developed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Julien Massot <julien.massot@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>