usb: gadget: uvc: Expose configuration name through video node

When utilising multiple instantiations of a UVC gadget on a composite
device, there is no clear method to link a particular configuration to
its respective video node.

Provide a means for identifying the correct video node by exposing the
name of the function configuration through sysfs.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>