bearer: allow subclasses to report disconnection

The new `mm_bearer_report_disconnection()' allows subclasses to notify about
being disconnected, so that every layer of inheritance can chain its own code
to reset the connection status.

This commit partially disables the logic included in commit 981222. Now
subclasses (actually, no one) are not allowed to g_object_set() the
MM_BEARER_STATUS property.

plugins/option/mm-broadband-bearer-hso.c

@@ -382,9 +382,7 @@ mm_broadband_bearer_hso_report_connection_status (MMBroadbandBearerHso *self,
             dial_3gpp_context_complete_and_free (ctx);
         } else {
             /* Just ensure we mark ourselves as being disconnected... */
-            g_object_set (self,
-                          MM_BEARER_STATUS, MM_BEARER_STATUS_DISCONNECTED,
-                          NULL);
+            mm_bearer_report_disconnection (MM_BEARER (self));
         }
         break;
     }