mailbox: Fix devm_ioremap_resource error detection code

devm_ioremap_resource returns an ERR_PTR value, not NULL,
on failure.

The Coccinelle semantic patch used to make this change is
as follows:
@@
expression e,e1;
statement S;
@@

*e = devm_ioremap_resource(...);
if (!e1) S

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/mailbox-sti.c

@@ -430,8 +430,8 @@ static int sti_mbox_probe(struct platform_device *pdev)
 
 	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
 	mdev->base = devm_ioremap_resource(&pdev->dev, res);
-	if (!mdev->base)
-		return -ENOMEM;
+	if (IS_ERR(mdev->base))
+		return PTR_ERR(mdev->base);
 
 	ret = of_property_read_string(np, "mbox-name", &mdev->name);
 	if (ret)