[media] uvcvideo: Fix empty packet statistic

The frame counters are inadvertently counting packets with content as
empty.

Fix it by correcting the logic expression

Fixes: 7bc5edb0

 [media] uvcvideo: Extract video stream statistics

Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>