media: v4l: subdev: Set sd->devnode before registering the subdev

The subdev's video device node was only assigned after registering the
device node in the system. While it is unlikely that a driver needed to
use this field in handling system calls to its file handle, there remains
a slim chance the devnode field remains NULL while the driver expects to
find a video node there.

Assign the devnode field before registering the device, and assign it back
to NULL if the registration failed.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>