drivers: modem: gsm_ppp: Enable start/stop

These changes enable applications to restart the networking stack which
was previously not possible without rebooting the device. This was a
major show-stopper because it made power management impossible, and
furthermore made it impossible to recover from a bad modem state without
rebooting.

This has been verified to work on a SIMCOM7600E modem, both with and
without CONFIG_GSM_MUX enabled.

Signed-off-by: Benjamin Lindqvist <benjamin.lindqvist@endian.se>