altera_uart: Don't use plain integer as NULL pointer

Fixes sparse warning.

Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>
Cc: Alan Cox <alan@linux.intel.com>
Acked-by: Tobias Klauser <tklauser@distanz.ch>

diff --git a/drivers/serial/altera_uart.c b/drivers/serial/altera_uart.c
index dfc1ecc..7212162 100644
--- a/drivers/serial/altera_uart.c
+++ b/drivers/serial/altera_uart.c
@@ -461,7 +461,7 @@
 	if (co->index < 0 || co->index >= CONFIG_SERIAL_ALTERA_UART_MAXPORTS)
 		return -EINVAL;
 	port = &altera_uart_ports[co->index].port;
-	if (port->membase == 0)
+	if (!port->membase)
 		return -ENODEV;
 
 	if (options)