[media] v4l2-subdev: Add description for core ioctl handlers

Warning(.//include/media/v4l2-subdev.h:183): No description found for parameter 'queryctrl'
Warning(.//include/media/v4l2-subdev.h:183): No description found for parameter 'g_ctrl'
Warning(.//include/media/v4l2-subdev.h:183): No description found for parameter 's_ctrl'
Warning(.//include/media/v4l2-subdev.h:183): No description found for parameter 'g_ext_ctrls'
Warning(.//include/media/v4l2-subdev.h:183): No description found for parameter 's_ext_ctrls'
Warning(.//include/media/v4l2-subdev.h:183): No description found for parameter 'try_ext_ctrls'
Warning(.//include/media/v4l2-subdev.h:183): No description found for parameter 'querymenu'
Warning(.//include/media/v4l2-subdev.h:183): No description found for parameter 'g_register'
Warning(.//include/media/v4l2-subdev.h:183): No description found for parameter 's_register'

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