usb: gadget: add a forward pointer from usb_function to its "instance"

We can have multiple usb_functions which origin is the same "instance".
Within one USB configuration there should be only one function of an
instance. This back pointer helps configfs to recoginze to which
instance a given usb_function belongs.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/include/linux/usb/composite.h b/include/linux/usb/composite.h
index 8c7a629..771de7a 100644
--- a/include/linux/usb/composite.h
+++ b/include/linux/usb/composite.h
@@ -161,6 +161,7 @@
 	/* internals */
 	struct list_head		list;
 	DECLARE_BITMAP(endpoints, 32);
+	const struct usb_function_instance *fi;
 };
 
 int usb_add_function(struct usb_configuration *, struct usb_function *);