media: staging/ipu7: Disallow source multiplexing

The IPU7 ISYS driver can't capture multiple streams on the same video
device. Disallow source multiplexing in the routes of the internal
subdev to reflect that limitation. As a result we can hardcode the
source stream to 0, simplifying the driver.

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