[media] uvcvideo: Delay initialization of XU controls

XU controls initialization requires querying the device for control
information. As some buggy UVC devices will crash when queried
repeatedly in a tight loop, delay XU controls initialization until first
use.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>