serial: zs: use NULL as a pointer, not 0

struct uart_port::membase is declared as a pointer. So it should be
initialized by NULL, not zero constant.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220421101708.5640-5-jslaby@suse.czSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/zs.c

@@ -981,7 +981,7 @@ static const char *zs_type(struct uart_port *uport)
 static void zs_release_port(struct uart_port *uport)
 {
 	iounmap(uport->membase);
-	uport->membase = 0;
+	uport->membase = NULL;
 	release_mem_region(uport->mapbase, ZS_CHAN_IO_SIZE);
 }