esp_modem.c: post PPP stop event when requested

When library users call esp_stop_ppp(), it is expected that the PPP link
is closed and the DCE is moved into command mode.

If the STOP event is not posted into the event loop, then the PPP stack
would not be turned off, resulting in the impossibility to switch into
command mode, as the modem is still sending PPP binary data.

Signed-off-by: Francesco Giancane <francesco.giancane@accenture.com>

Merges https://github.com/espressif/esp-idf/pull/4642