usb: gadget: uvc: Pick up custom string descriptor IDs

usb: gadget: uvc: Pick up custom string descriptor IDs

If any custom string descriptors have been linked to from the
extension unit, pick up the string ID that was returned when
the strings were attached to the composite dev and use it to
set the iExtension field of the Extension Unit Descriptor.

Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Link: https://lore.kernel.org/r/20230206161802.892954-10-dan.scally@ideasonboard.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>