USB: serial: quatech2: fix sleep-while-atomic in close

The write URB was being killed using the synchronous interface while
holding a spin lock in close().

Simply drop the lock and busy-flag update, something which would have
been taken care of by the completion handler if the URB was in flight.

Fixes: f7a33e608d9a ("USB: serial: add quatech2 usb to serial driver")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/quatech2.c

@@ -408,16 +408,12 @@ static void qt2_close(struct usb_serial_port *port)
 {
 	struct usb_serial *serial;
 	struct qt2_port_private *port_priv;
-	unsigned long flags;
 	int i;
 
 	serial = port->serial;
 	port_priv = usb_get_serial_port_data(port);
 
-	spin_lock_irqsave(&port_priv->urb_lock, flags);
 	usb_kill_urb(port_priv->write_urb);
-	port_priv->urb_in_use = false;
-	spin_unlock_irqrestore(&port_priv->urb_lock, flags);
 
 	/* flush the port transmit buffer */
 	i = usb_control_msg(serial->dev,