broadband-modem-qmi: don't pass full reference to signal handlers

As in the case of the timeouts, we should not pass a full reference as
data to the signal handler as we don't know how many times we'll
receive the signal or if we'll ever receive it.

We already make sure that the signal handler is disconnected when the
operation ends, so we can be sure the self pointer is valid while the
signal handler is connected.

src/mm-broadband-modem-qmi.c

@@ -1793,7 +1793,6 @@ power_event_report_indication_cb (QmiClientDms                      *client,
                              MM_CORE_ERROR_FAILED,
                              "Invalid power indication received");
         dms_set_operating_mode_complete (self, error);
-        g_object_unref (self);
         return;
     }
 
@@ -1804,7 +1803,6 @@ power_event_report_indication_cb (QmiClientDms                      *client,
     if (mode == state) {
         mm_obj_dbg (self, "Requested mode and mode received by indication matched");
         dms_set_operating_mode_complete (self, NULL);
-        g_object_unref (self);
         return;
     }
 
@@ -1812,7 +1810,6 @@ power_event_report_indication_cb (QmiClientDms                      *client,
                          MM_CORE_ERROR_FAILED,
                          "Requested mode and mode received by indication did not match");
     dms_set_operating_mode_complete (self, error);
-    g_object_unref (self);
 }
 
 static void
@@ -1932,7 +1929,7 @@ dms_set_event_report_operating_mode_activate_ready (QmiClientDms        *client,
         g_signal_connect (client,
                           "event-report",
                           G_CALLBACK (power_event_report_indication_cb),
-                          g_object_ref (self));
+                          self);
 
     g_assert (self->priv->set_operating_mode_task != NULL);
     mm_obj_dbg (self, "Power operation is pending");