serial/imx: make devdata member point to const data

This is only cosmetic for now. In case that

	http://mid.gmane.org/1335171381-24869-1-git-send-email-u.kleine-koenig@pengutronix.de

will be applied, it fixes a warning

	drivers/tty/serial/imx.c: In function 'serial_imx_probe_dt':
	drivers/tty/serial/imx.c:1430:17: warning: assignment discards 'const' qualifier from pointer target type [enabled by default]

though.
Signed-off-by: NUwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -207,7 +207,7 @@ struct imx_port {
 	unsigned short		trcv_delay; /* transceiver delay */
 	struct clk		*clk_ipg;
 	struct clk		*clk_per;
-	struct imx_uart_data	*devdata;
+	const struct imx_uart_data *devdata;
 };
 
 struct imx_port_ucrs {