[media] uvc gadget: set device_caps in querycap

The V4L2 core will warn if this is not done. Unfortunately this driver
wasn't updated.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>