Fix new devices on restart, if serial not yet available.

[shivan]

This is how I fixed the extra devices. At least for me I did not get any duplicated devices since some restarts.

So this could solve #23

But you first have to cleanup once, or you'll get a third device ;-)

[cofade]

This is the right fix. The core problem is that the old code tied the unique_id to data that isn't available yet at startup, so any slow or offline inverter triggered a new entity. IP-octet as stable fallback is a clean solution. Running a variation of this locally, no more duplicates. One-time cleanup is a reasonable tradeoff. Please merge this.

[KoljaWindeler]

thanks @shivan I'll give it a try here and report back