usb: set USB 3.1 roothub device speed to USB_SPEED_SUPER_PLUS

BugLink: http://bugs.launchpad.net/bugs/1519623

A hcd roothub that supports HCD_USB31 is running at SuperSpeedPlus speed
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 5f9c3a66)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/usb/core/hcd.c

@@ -2778,9 +2778,11 @@ int usb_add_hcd(struct usb_hcd *hcd,
 		rhdev->speed = USB_SPEED_WIRELESS;
 		break;
 	case HCD_USB3:
-	case HCD_USB31:
 		rhdev->speed = USB_SPEED_SUPER;
 		break;
+	case HCD_USB31:
+		rhdev->speed = USB_SPEED_SUPER_PLUS;
+		break;
 	default:
 		retval = -EINVAL;
 		goto err_set_rh_speed;