[media] em28xx: fix tuner/frequency handling

v4l2-compliance found problems with frequency clamping that wasn't
reported correctly and missing tuner index checks.
Also removed unnecessary tuner type checks (these are now done by the
v4l2 core).

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