[media] uvcvideo: Fix reading the current exposure value of UVC

V4L2_CID_EXPOSURE_ABSOLUTE property does not return an updated value when
autoexposure (V4L2_CID_EXPOSURE_AUTO) is turned on. This patch fixes this
issue by adding the UVC_CTRL_FLAG_AUTO_UPDATE flag.

Tested on a C920 camera.

Signed-off-by: Anton V. Shokurov <shokurov.anton.v@yandex.ru>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>