V4L/DVB: uvcvideo: Flag relative controls as write-only

The UVC relative controls (exposure time, iris, focus, zoom, pan/tilt)
are write-only (despite the UVC specification stating that the GET_CUR
request is mandatory). Mark the controls as such, and report the related
V4L2 controls V4L2_CTRL_FLAG_WRITE_ONLY.

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