core: ensure new available connections generate a ProperiesChanged signal (bgo #697743)

The TRUE return got missed in the original commit of the AvailableConnections
code.  _try_add_available_connection() returns TRUE if the connection was
added and FALSE if it wasn't, to allow the caller to optionally emit the
PropertiesChanged signal.  Only TRUE was never returned...

src/nm-device.c

@@ -5783,6 +5783,7 @@ _try_add_available_connection (NMDevice *self, NMConnection *connection)
 			g_hash_table_insert (NM_DEVICE_GET_PRIVATE (self)->available_connections,
 					             g_object_ref (connection),
 					             GUINT_TO_POINTER (1));
+			return TRUE;
 		}
 	}
 	return FALSE;