sierra: fix warnings with -Wswitch-default

plugins/sierra/mm-broadband-bearer-sierra.c

@@ -434,6 +434,9 @@ dial_3gpp_context_step (GTask *task)
                                g_object_unref);
         g_object_unref (task);
         return;
+
+    default:
+        g_assert_not_reached ();
     }
 }
 

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

@@ -1446,9 +1446,10 @@ cdma_automatic_activation_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
-    }
 
+    default:
         g_assert_not_reached ();
+    }
 }
 
 static void
@@ -1601,9 +1602,10 @@ cdma_manual_activation_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
-    }
 
+    default:
         g_assert_not_reached ();
+    }
 }
 
 static void