Fixes missing translation of enums when reading the security mode from a simplelink profile.

[balazsracz]

The external enum is not the same as the internal enum. We have translation functions security_type_from_simplelink and security_type_to_simplelink. We forgot to call this function on one of the codepaths.

Updates the setup_ap function to be able to be called with nullptr as ssid and security key, to just update the security type.