[review] Fix MM_SIGNAL_UNKNOWN usage

[Aleksander Morgado]

Hey,

The following MR fixes how MM_SIGNAL_UNKNOWN is used to report unset values:

https://gitlab.freedesktop.org/mobile-broadband/ModemManager/merge_requests/24

I thought of deprecating the symbol and adding a new one instead,
given that we're changing the actual value of the symbol, but that
would even be more confusing for clients that may be using this
symbol. Changing the value is the easiest and cleanest way to fix it.

-- 
Aleksander
https://aleksander.es