NAS gateway: fix notification initialization (#8920)

Co-authored-by: Harshavardhana <harsha@minio.io>

cmd/notification.go

@@ -704,7 +704,7 @@ func (sys *NotificationSys) Init(buckets []BucketInfo, objAPI ObjectLayer) error
 	}
 
 	// In gateway mode, notifications are not supported.
-	if globalIsGateway {
+	if globalIsGateway && !objAPI.IsNotificationSupported() {
 		return nil
 	}