tty/serial: digicolor: fix possible null-ptr-deref in digicolor_uart_probe()

It will cause null-ptr-deref when using 'res', if platform_get_resource()
returns NULL, so move using 'res' after devm_ioremap_resource() that
will check it to avoid null-ptr-deref.
And use devm_platform_get_and_ioremap_resource() to simplify code.

Fixes: 5930cb35 ("serial: driver for Conexant Digicolor USART")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Baruch Siach <baruch@tkos.co.il>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20220505124621.1592697-1-yangyingliang@huawei.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/digicolor-usart.c

@@ -471,11 +471,10 @@ static int digicolor_uart_probe(struct platform_device *pdev)
 	if (IS_ERR(uart_clk))
 		return PTR_ERR(uart_clk);
 
-	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
-	dp->port.mapbase = res->start;
-	dp->port.membase = devm_ioremap_resource(&pdev->dev, res);
+	dp->port.membase = devm_platform_get_and_ioremap_resource(pdev, 0, &res);
 	if (IS_ERR(dp->port.membase))
 		return PTR_ERR(dp->port.membase);
+	dp->port.mapbase = res->start;
 
 	irq = platform_get_irq(pdev, 0);
 	if (irq < 0)