usb: gadget: uvc: configfs: Drop leaked references to config items

Some of the .allow_link() and .drop_link() operations implementations
call config_group_find_item() and then leak the reference to the
returned item. Fix this by dropping those references where needed.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>