[media] vivid: report only one frameinterval

The vivid driver reports a range of frame intervals for non-webcams, when in fact
the frame interval is fixed for those inputs as it depends on the DV timings or
standard. Just report the single discrete frame interval instead.

Caught by v4l2-compliance.

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