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

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: f9c56cd (usb: gadget: Clear usb_endpoint_descriptor
	inside the struct usb_ep on disable)
Cc: <stable@vger.kernel.org> # v3.4+
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/function/f_acm.c

@@ -433,12 +433,12 @@ static int acm_set_alt(struct usb_function *f, unsigned intf, unsigned alt)
 			dev_vdbg(&cdev->gadget->dev,
 				 "reset acm control interface %d\n", intf);
 			usb_ep_disable(acm->notify);
-		} else {
-			dev_vdbg(&cdev->gadget->dev,
-				 "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;