Fix the following 'make htmldocs' warnings:

  .//include/media/v4l2-dv-timings.h:147: warning: No description found for parameter 'frame_height'
  .//include/media/v4l2-dv-timings.h:147: warning: No description found for parameter 'hfreq'
  .//include/media/v4l2-dv-timings.h:147: warning: No description found for parameter 'vsync'
  .//include/media/v4l2-dv-timings.h:147: warning: No description found for parameter 'active_width'
  .//include/media/v4l2-dv-timings.h:147: warning: No description found for parameter 'polarities'
  .//include/media/v4l2-dv-timings.h:147: warning: No description found for parameter 'interlaced'
  .//include/media/v4l2-dv-timings.h:147: warning: No description found for parameter 'fmt'
  .//include/media/v4l2-dv-timings.h:171: warning: No description found for parameter 'frame_height'
  .//include/media/v4l2-dv-timings.h:171: warning: No description found for parameter 'hfreq'
  .//include/media/v4l2-dv-timings.h:171: warning: No description found for parameter 'vsync'
  .//include/media/v4l2-dv-timings.h:171: warning: No description found for parameter 'polarities'
  .//include/media/v4l2-dv-timings.h:171: warning: No description found for parameter 'interlaced'
  .//include/media/v4l2-dv-timings.h:171: warning: No description found for parameter 'aspect'
  .//include/media/v4l2-dv-timings.h:171: warning: No description found for parameter 'fmt'
  .//include/media/v4l2-dv-timings.h:184: warning: No description found for parameter 'hor_landscape'
  .//include/media/v4l2-dv-timings.h:184: warning: No description found for parameter 'vert_portrait'

Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>