[media] cx23885: fix UNSET/TUNER_ABSENT confusion

Sometimes dev->tuner_type is compared to UNSET, sometimes to TUNER_ABSENT,
but these defines have different values.

Standardize to TUNER_ABSENT.

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