[media] uvcvideo: Disable hardware timestamps by default

The hardware timestamping implementation has been reported as not
working correctly on at least the Logitech C920. Until this can be
fixed, disable it by default.

Reported-by: Peter Rabbitson <rabbit@rabbit.us>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>