mailbox: th1520: Fix a NULL vs IS_ERR() bug

The devm_ioremap() function doesn't return error pointers, it returns
NULL.  Update the error checking to match.

Fixes: 5d4d263 ("mailbox: Introduce support for T-head TH1520 Mailbox driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Michal Wilczynski <m.wilczynski@samsung.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

Author: Dan Carpenter

drivers/mailbox/mailbox-th1520.c

@@ -387,8 +387,10 @@ static void __iomem *th1520_map_mmio(struct platform_device *pdev,
 
 	mapped = devm_ioremap(&pdev->dev, res->start + offset,
 			      resource_size(res) - offset);
-	if (IS_ERR(mapped))
+	if (!mapped) {
 		dev_err(&pdev->dev, "Failed to map resource: %s\n", res_name);
+		return ERR_PTR(-ENOMEM);
+	}
 
 	return mapped;
 }