[media] bttv: do not switch to the radio tuner unless it is accessed

Just opening the radio tuner should not cause a switch to the radio tuner.
Only after calling g/s_tuner or g/s_frequency should this happen.
This prevents audio being unmuted as soon as the driver is loaded because
some process opens /dev/radioX just to see what sort of node it is, which
switches on the radio tuner and unmutes audio.
This code can be improved further by actually keeping track of who owns the
tuner and returning -EBUSY if switching tuner modes will cause problems.
But for now just fix the annoying case where on boot the radio turns on
automatically.

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