fix(modem): Refine mode switch data->command

* netif.stop() moved after setting the transition callback
* send PPP escape sequence if enabled before waiting for transition to
complete
* add newline character before sync() command (after the "+++")

Closes https://github.com/espressif/esp-protocols/issues/692

components/esp_modem/src/esp_modem_netif_linux.cpp

@@ -52,7 +52,6 @@ void Netif::start()
 
 void Netif::stop()
 {
-    ppp_dte->set_read_cb(nullptr);
     signal.clear(PPP_STARTED);
 }