tty: serial: altera_jtaguart: Don't use plain integer as NULL pointer

This fixes a sparse warning.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
diff --git a/drivers/tty/serial/altera_jtaguart.c b/drivers/tty/serial/altera_jtaguart.c
index f9b49b5..0c0a8b6 100644
--- a/drivers/tty/serial/altera_jtaguart.c
+++ b/drivers/tty/serial/altera_jtaguart.c
@@ -384,7 +384,7 @@
 	if (co->index < 0 || co->index >= ALTERA_JTAGUART_MAXPORTS)
 		return -EINVAL;
 	port = &altera_jtaguart_ports[co->index].port;
-	if (port->membase == 0)
+	if (port->membase == NULL)
 		return -ENODEV;
 	return 0;
 }