mailbox: mailbox-test: Fix debugfs_create_dir error checking

The debugfs_create_dir() function returns ERR_PTR() on error, not NULL.
The current null-check fails to catch errors.

Use IS_ERR() to correctly check for errors.

Fixes: 8ea4484 ("mailbox: Add generic mechanism for testing Mailbox Controllers")
Signed-off-by: Haotian Zhang <vulab@iscas.ac.cn>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

Author: ISCAS-Vulab

drivers/mailbox/mailbox-test.c

@@ -268,7 +268,7 @@ static int mbox_test_add_debugfs(struct platform_device *pdev,
 		return 0;
 
 	tdev->root_debugfs_dir = debugfs_create_dir(dev_name(&pdev->dev), NULL);
-	if (!tdev->root_debugfs_dir) {
+	if (IS_ERR(tdev->root_debugfs_dir)) {
 		dev_err(&pdev->dev, "Failed to create Mailbox debugfs\n");
 		return -EINVAL;
 	}