[media] uvcvideo: Ignore GET_RES error for XU controls

GET_RES request support is mandatory for extension units, but some
cameras still choke on it (one example is the Logitech QuickCam PTZ that
returns a single byte for the PTZ relative control instead of four).

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