[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>

drivers/media/usb/uvc/uvc_v4l2.c

@@ -1133,6 +1133,9 @@ static int uvc_ioctl_enum_frameintervals(struct file *file, void *fh,
 		uvc_simplify_fraction(&fival->discrete.numerator,
 			&fival->discrete.denominator, 8, 333);
 	} else {
+		if (fival->index)
+			return -EINVAL;
+
 		fival->type = V4L2_FRMIVAL_TYPE_STEPWISE;
 		fival->stepwise.min.numerator = frame->dwFrameInterval[0];
 		fival->stepwise.min.denominator = 10000000;