novatel: fix warnings with -Wswitch-default

novatel/mm-broadband-modem-novatel.c: In function ‘cdma_activation_step’:
  novatel/mm-broadband-modem-novatel.c:1045:5: error: switch missing default case [-Werror=switch-default]
   1045 |     switch (ctx->step) {
        |     ^~~~~~

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

@@ -1101,9 +1101,10 @@ cdma_activation_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
-    }
 
-    g_assert_not_reached ();
+    default:
+        g_assert_not_reached ();
+    }
 }
 
 static void