serial: imx: Use NULL as the last argument of add_preferred_console()

Commit f7d2c0bb (serial: i.MX: evaluate linux,stdout-path property) introduced
the following sparse warning:

drivers/tty/serial/imx.c:1916:77: warning: Using plain integer as NULL pointer

Pass NULL as the last argument of add_preferred_console() instead of zero.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -1913,7 +1913,8 @@ static int serial_imx_probe_dt(struct imx_port *sport,
 	sport->devdata = of_id->data;
 
 	if (of_device_is_stdout_path(np))
-		add_preferred_console(imx_reg.cons->name, sport->port.line, 0);
+		add_preferred_console(imx_reg.cons->name, sport->port.line,
+				      NULL);
 
 	return 0;
 }