[media] uvcvideo: Validate index during step-wise frame intervals enumeration

Frame intervals exposed as an interval and step (so-called step-wise)
are restricted by the V4L2 API to a single enumeration entry. Return an
error when the index is not zero.

Reported-by: Alexey Smirnoff <fling@member.fsf.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
[mchehab@osg.samsung.com: add a missing collon at the end of the return
 statement, in order to avoid compilation breakage]
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>