dtivers: ntb: fix parameter check in perf_setup_dbgfs()

Make IS_ERR() judge the debugfs_create_dir() function return
in perf_setup_dbgfs().

Signed-off-by: Minjie Du <duminjie@vivo.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>

Author: Minjie Du

drivers/ntb/test/ntb_perf.c

@@ -1355,7 +1355,7 @@ static void perf_setup_dbgfs(struct perf_ctx *perf)
 	struct pci_dev *pdev = perf->ntb->pdev;
 
 	perf->dbgfs_dir = debugfs_create_dir(pci_name(pdev), perf_dbgfs_topdir);
-	if (!perf->dbgfs_dir) {
+	if (IS_ERR(perf->dbgfs_dir)) {
 		dev_warn(&perf->ntb->dev, "DebugFS unsupported\n");
 		return;
 	}