V4L/DVB (3256): When in radio mode ignore VIDIOC_G_TUNER

- Detect when AUDC_SET_RADIO is called.
- When in radio mode ignore VIDIOC_G_TUNER.

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