[media] cx23885: Don't duplicate xc4000 entry for radio

All radio tuners in cx23885 driver using same address for radio and
tuner, so there is no need to probe it twice for same tuner and we can
use radio_type UNSET.

Be aware radio support in cx23885 is not yet committed, so this is only
minor fix for future support.

Signed-off-by: Miroslav Slugen <thunder.mmm@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>