[media] v4l: uvcvideo: Add support for pan/tilt speed controls

Map V4L2_CID_TILT_SPEED and V4L2_CID_PAN_SPEED to the standard UVC
CT_PANTILT_RELATIVE_CONTROL terminal control request.

Tested by plugging a Logitech ConferenceCam C3000e USB camera
and controlling pan/tilt from the userspace using the VIDIOC_S_CTRL ioctl.
Verified that it can pan and tilt at the same time in both directions.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Pawel Osciak <posciak@chromium.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>