port-probe: duplicate amount of AT probing attempts

We originally did only 3 attempts every 3s, so the maximum probing time for a given TTY port looking for AT capabilities was 9 seconds. We now duplicate the amount of times, so up to 18s of port probing, which is quite a lot more than before, but unfortunately it's needed because most new modems take a lot of time to boot up internally.

The way to avoid this long probing time is to make sure port type hints for the possible AT ports are set correctly.