OPP: fix error checking in opp_migrate_dentry()

Since commit ff9fb72 ("debugfs: return error values, not NULL")
changed return value of debugfs_rename() in error cases from %NULL to
%ERR_PTR(-ERROR), we should also check error values instead of NULL.

Fixes: ff9fb72 ("debugfs: return error values, not NULL")
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Link: https://lore.kernel.org/r/20230208040037.60305-1-zhengqi.arch@bytedance.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Qi Zheng

drivers/opp/debugfs.c

@@ -235,7 +235,7 @@ static void opp_migrate_dentry(struct opp_device *opp_dev,
 
 	dentry = debugfs_rename(rootdir, opp_dev->dentry, rootdir,
 				opp_table->dentry_name);
-	if (!dentry) {
+	if (IS_ERR(dentry)) {
 		dev_err(dev, "%s: Failed to rename link from: %s to %s\n",
 			__func__, dev_name(opp_dev->dev), dev_name(dev));
 		return;