simtech: fix error reporting in 3gpp unsolicited events enabling

(cherry picked from commit 5ee58513)

plugins/simtech/mm-broadband-modem-simtech.c

@@ -244,6 +244,7 @@ parent_enable_unsolicited_events_ready (MMIfaceModem3gpp *self,
         g_simple_async_result_take_error (simple, error);
         g_simple_async_result_complete (simple);
         g_object_unref (simple);
+        return;
     }
 
     /* Our own enable now */