Fix Wifi Country Code being reset on reboot

System attempts to be clever and set WiFi Region Code via MCC country code.
Which messes stuff up if you're not in a "default" country like US,GB,AU etc.
It also overwrites any choice the user has made via the WiFi Advanced Settings
menu. Ie. user with "US" MCC code, selects GB (to get 13 channels), will be
reset to US.

Add check to see if country code has already been set, if so, don't reset to
MCC value. This should enable default cases to still work correctly on first
boot, and non default values to be corrected and persisted using the Advanced
WiFi Settings without being overwritten during boot.