V4L/DVB (8107): cx18: improve support for the Raptor board.

- add radio definition
- reset the audio firmware (required for this board, harmless for the
others)

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>