imx6: ventana: fix system-serial dt property

Fix typo in setting of system-serial property causing the prop len to be
off by 1.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>

board/gateworks/gw_ventana/gw_ventana.c

@@ -1277,7 +1277,7 @@ void ft_board_setup(void *blob, bd_t *bd)
 
 	/* board serial number */
 	fdt_setprop(blob, 0, "system-serial", getenv("serial#"),
-		    strlen(getenv("serial#") + 1));
+		    strlen(getenv("serial#")) + 1);
 
 	/* board (model contains model from device-tree) */
 	fdt_setprop(blob, 0, "board", info->model,