usb: gadget: function: acm: make f_acm pass USB20CV Chapter9

commit 52ec49a5 upstream.

During Halt Endpoint Test, our interrupt endpoint
will be disabled, which will clear out ep->desc
to NULL. Unless we call config_ep_by_speed() again,
we will not be able to enable this endpoint which
will make us fail that test.

Fixes: f9c56cdd (usb: gadget: Clear usb_endpoint_descriptor
	inside the struct usb_ep on disable)
Signed-off-by: Felipe Balbi <balbi@ti.com>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/usb/gadget/f_acm.c

@@ -430,11 +430,12 @@ static int acm_set_alt(struct usb_function *f, unsigned intf, unsigned alt)
 		if (acm->notify->driver_data) {
 			VDBG(cdev, "reset acm control interface %d\n", intf);
 			usb_ep_disable(acm->notify);
-		} else {
-			VDBG(cdev, "init acm ctrl interface %d\n", intf);
+		}
+
+		if (!acm->notify->desc)
 			if (config_ep_by_speed(cdev->gadget, f, acm->notify))
 				return -EINVAL;
-		}
+
 		usb_ep_enable(acm->notify);
 		acm->notify->driver_data = acm;