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>