usb: gadget: move bind() callback back to struct usb_gadget_driver

This partly reverts 07a18bd7 ("usb gadget: don't save bind callback in
struct usb_composite_driver") and fixes new drivers. The section missmatch
problems was solved by whitelisting bind callback in modpost.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/dbgp.c

@@ -405,6 +405,7 @@ fail:
 static __refdata struct usb_gadget_driver dbgp_driver = {
 	.function = "dbgp",
 	.max_speed = USB_SPEED_HIGH,
+	.bind = dbgp_bind,
 	.unbind = dbgp_unbind,
 	.setup = dbgp_setup,
 	.disconnect = dbgp_disconnect,
@@ -416,7 +417,7 @@ static __refdata struct usb_gadget_driver dbgp_driver = {
 
 static int __init dbgp_init(void)
 {
-	return usb_gadget_probe_driver(&dbgp_driver, dbgp_bind);
+	return usb_gadget_probe_driver(&dbgp_driver);
 }
 
 static void __exit dbgp_exit(void)