serial: 8250: longson: Fix NULL vs IS_ERR() bug in probe

The devm_platform_get_and_ioremap_resource() function never returns
NULL, it returns error pointers.  Fix the error checking to match.

Fixes: 25e95d7 ("serial: 8250: Add Loongson uart driver support")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://patch.msgid.link/aSsIa3KdAlXh5uQC@stanley.mountain
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Dan Carpenter

drivers/tty/serial/8250/8250_loongson.c

@@ -128,8 +128,8 @@ static int loongson_uart_probe(struct platform_device *pdev)
 	port->private_data = priv;
 
 	port->membase = devm_platform_get_and_ioremap_resource(pdev, 0, &priv->res);
-	if (!port->membase)
-		return -ENOMEM;
+	if (IS_ERR(port->membase))
+		return PTR_ERR(port->membase);
 
 	port->mapbase = priv->res->start;
 	port->mapsize = resource_size(priv->res);