esp_modem: Return true from on_data callback in data mode

The callback should return false only if there is a problem with the received data.
The updated UART Terminal implementation cannot longer clear is callback.
It is DTE's responsibility to clear the callback.

components/esp_modem/src/esp_modem_netif.cpp

@@ -87,7 +87,7 @@ void Netif::start()
 {
     ppp_dte->set_read_cb([this](uint8_t *data, size_t len) -> bool {
         receive(data, len);
-        return false;
+        return true;
     });
     esp_netif_action_start(driver.base.netif, nullptr, 0, nullptr);
     signal.set(PPP_STARTED);