[media] v4l2-subdev: add remaining argument descriptions

Warning(.//include/media/v4l2-subdev.h:203): No description found for parameter 'ioctl'
Warning(.//include/media/v4l2-subdev.h:203): No description found for parameter 'compat_ioctl32'
Warning(.//include/media/v4l2-subdev.h:203): No description found for parameter 'subscribe_event'
Warning(.//include/media/v4l2-subdev.h:203): No description found for parameter 'unsubscribe_event'
Warning(.//include/media/v4l2-subdev.h:273): No description found for parameter 's_stream'
Warning(.//include/media/v4l2-subdev.h:407): No description found for parameter 's_stream'
Warning(.//include/media/v4l2-subdev.h:623): No description found for parameter 'link_validate'
Warning(.//include/media/v4l2-subdev.h:623): No description found for parameter 'set_frame_desc'

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