Fix start_discovery error return value
diff --git a/src/adapter.c b/src/adapter.c
index 47ee26a..2ff59a0 100644
--- a/src/adapter.c
+++ b/src/adapter.c
@@ -1151,7 +1151,7 @@
 		err = adapter_ops->start_scanning(adapter->dev_id);
 		break;
 	default:
-		err = -1;
+		err = -EINVAL;
 	}
 
 	return err;