V4L/DVB (11835): uvcvideo: Parse frame descriptors with non-continuous indexes.

The UVC specification requires frame descriptors indexes to range from 1 to
the number of frame descriptors. At least some Hercules Dualpix Infinite
webcams erroneously use non-continuous index ranges. Make the driver support
them.

Signed-off-by: Laurent Pinchart <laurent.pinchart@skynet.be>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>