serial: 8250_bcm7271: use NULL to initialized a null pointer

Pointer membase is currently being in initialized with zero rather
than NULL. Fix this.

Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20210719095533.14017-1-colin.king@canonical.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_bcm7271.c

@@ -941,7 +941,7 @@ static int brcmuart_probe(struct platform_device *pdev)
 	struct clk *baud_mux_clk;
 	struct uart_8250_port up;
 	struct resource *irq;
-	void __iomem *membase = 0;
+	void __iomem *membase = NULL;
 	resource_size_t mapbase = 0;
 	u32 clk_rate = 0;
 	int ret;