media: Documentation: Initialisation finishes before subdev registration

Document that sub-device initialisation needs to complete before the async
sub-device is registered as there is no further driver action needed
before the sensor becomes accessible via the UAPI.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>