USB: fix errornous assumption in the usb serial framework revealed by iuu_phoenix

the iuu_phoenix driver submits another URB from a completion handler.
This dictates a certain order of calls to usb_kill_urb() in kill_traffic().
As other drivers may do it the other way round, we need to use both
orders in kill_traffic().
This patch does so and should be merged before iuu_phoenix is merged.
Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/usb-serial.c

@@ -578,6 +578,17 @@ static void kill_traffic(struct usb_serial_port *port)
 {
 	usb_kill_urb(port->read_urb);
 	usb_kill_urb(port->write_urb);
+	/*
+	 * This is tricky.
+	 * Some drivers submit the read_urb in the
+	 * handler for the write_urb or vice versa
+	 * this order determines the order in which
+	 * usb_kill_urb() must be used to reliably
+	 * kill the URBs. As it is unknown here,
+	 * both orders must be used in turn.
+	 * The call below is not redundant.
+	 */
+	usb_kill_urb(port->read_urb);
 	usb_kill_urb(port->interrupt_in_urb);
 	usb_kill_urb(port->interrupt_out_urb);
 }