[media] Docbook: Fix description of struct media_devnode

Warning(.//include/media/media-devnode.h:80): No description found for parameter 'fops'
Warning(.//include/media/media-devnode.h:80): No description found for parameter 'dev'
Warning(.//include/media/media-devnode.h:80): No description found for parameter 'cdev'
Warning(.//include/media/media-devnode.h:80): No description found for parameter 'release'

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>