media: media-devnode: fill in media chardev kobject to ease debugging

The media chardev kobject has no name, which made it hard to
debug when kobject debugging is turned on.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>