usb, composite: after unregister gadget driver set composite to NULL

Without this, second usb_composite_register() call fails always
with -EINVAL.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Marek Vasut <marex@denx.de>

drivers/usb/gadget/composite.c

@@ -1098,4 +1098,5 @@ void usb_composite_unregister(struct usb_composite_driver *driver)
 	if (composite != driver)
 		return;
 	usb_gadget_unregister_driver(&composite_driver);
+	composite = NULL;
 }