V4L/DVB (3255): When in radio mode don't do anything with VIDIOC_G_TUNER.

- When in radio mode don't do anything with VIDIOC_G_TUNER.
Allow other devices to fill this.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@brturbo.com.br>