USB: serial: keyspan_pda: fix tx-unthrottle use-after-free

The driver's transmit-unthrottle work was never flushed on disconnect,
something which could lead to the driver port data being freed while the
unthrottle work is still scheduled.

Fix this by cancelling the unthrottle work when shutting down the port.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/keyspan_pda.c

@@ -647,8 +647,12 @@ static int keyspan_pda_open(struct tty_struct *tty,
 }
 static void keyspan_pda_close(struct usb_serial_port *port)
 {
+	struct keyspan_pda_private *priv = usb_get_serial_port_data(port);
+
 	usb_kill_urb(port->write_urb);
 	usb_kill_urb(port->interrupt_in_urb);
+
+	cancel_work_sync(&priv->unthrottle_work);
 }