media: uvcvideo: Allow entity-defined get_info and get_cur

Allows controls to get their properties and current value
from an entity-defined function instead of via a query to the USB
device.

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>