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: NKieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>