[media] uvcvideo: Correct speed testing

Allow for SS+ USB devices.

Signed-off-by: Oliver Neukum <ONeukum@suse.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>