[media] bttv: fix s_tuner for radio

Fix typo: g_tuner should have been s_tuner.

Tested with a bttv card.

Cc: stable@kernel.org
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>