[media] cx88: consistently use UNSET for absent tuner

Don't mix UNSET and TUNER_ABSENT: you have to pick one or the other. For
this driver selecting UNSET to represent an absent tuner resulting in
the fewest changes.

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