media: Documentation: Document -ENXIO for VIDIOC_SUBDEV_S_ROUTING

media: Documentation: Document -ENXIO for VIDIOC_SUBDEV_S_ROUTING

Document that -ENXIO is returned when the user tries to set a routing
configuration not supported by the hardware (or rather the driver). The
documentation details the exact cases of this, besides -EINVAL that is
already documented for VIDIOC_SUBDEV_S_ROUTING.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>