[media] cx88: fix: don't duplicate xc4000 entry for radio

All radio tuners in cx88 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, this also fix broken radio since kernel 2.6.39-rc1
for those tuners.

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