[media] bttv: disable g/s_tuner and g/s_freq when no tuner present, fix return codes

[media] bttv: disable g/s_tuner and g/s_freq when no tuner present, fix return codes

If no tuner is present, then disable the tuner and frequency ioctls.
We can remove a number of checks from those ioctls testing for the presence
of a tuner.
Also remove some tuner type checks (now done by the core) and fix an
error return when the prio check fails.
Finally some 'unlikely' statements are removed since those only make sense
in tightly often executed loops, otherwise they just clutter up the code.

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