telit: fix warnings with -Wswitch-default

telit/mm-broadband-modem-telit.c: In function ‘qss_setup_step’:
  telit/mm-broadband-modem-telit.c:616:5: error: switch missing default case [-Werror=switch-default]
    616 |     switch (ctx->step) {
        |     ^~~~~~

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

@@ -664,6 +664,9 @@ qss_setup_step (GTask *task)
                 g_task_return_boolean (task, TRUE);
             g_object_unref (task);
             break;
+
+        default:
+            g_assert_not_reached ();
     }
 }