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

Warning(.//include/media/v4l2-subdev.h:224): No description found for parameter 's_radio'
Warning(.//include/media/v4l2-subdev.h:224): No description found for parameter 's_frequency'
Warning(.//include/media/v4l2-subdev.h:224): No description found for parameter 'enum_freq_bands'
Warning(.//include/media/v4l2-subdev.h:224): No description found for parameter 'g_tuner'
Warning(.//include/media/v4l2-subdev.h:224): No description found for parameter 'g_modulator'
Warning(.//include/media/v4l2-subdev.h:224): No description found for parameter 's_modulator'

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