[media] cx18: Fix FM radio

The cx18 driver was not setting up the analog tuner driver to enable
FM radio.  This change fixes that.  Thanks go to Ian Armstrong for
reporting and fixing the analogous problem in ivtv.

Signed-off-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>