usb: gadget: uvc: Allow linking XUs to string descriptors

Add .allow_link() and .drop_link() callbacks to allow users to link
an extension unit descriptor to a string descriptor.

Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Link: https://lore.kernel.org/r/20230206161802.892954-9-dan.scally@ideasonboard.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>