usb: musb: fix hanging when rmmod gadget driver

If we try to modprobe a second gadget driver before
rmmoding the first one, the reference for the first
gadget driver would get NULLed avoiding usb to change
gadget drivers later.

Cc: David Brownell <david-b@pacbell.net>
Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/musb/musb_gadget.c

@@ -1710,17 +1710,15 @@ int usb_gadget_register_driver(struct usb_gadget_driver *driver)
 
 	spin_unlock_irqrestore(&musb->lock, flags);
 
-	if (retval == 0)
+	if (retval == 0) {
 		retval = driver->bind(&musb->g);
-	if (retval != 0) {
-		DBG(3, "bind to driver %s failed --> %d\n",
-			driver->driver.name, retval);
-		musb->gadget_driver = NULL;
-		musb->g.dev.driver = NULL;
-	}
+		if (retval != 0) {
+			DBG(3, "bind to driver %s failed --> %d\n",
+					driver->driver.name, retval);
+			musb->gadget_driver = NULL;
+			musb->g.dev.driver = NULL;
+		}
 
-	/* start peripheral and/or OTG engines */
-	if (retval == 0) {
 		spin_lock_irqsave(&musb->lock, flags);
 
 		/* REVISIT always use otg_set_peripheral(), handling