iwlwifi: disable more MEI (Management Engine) code

It is unclear why this hasn't bothered anyone in months; I can only assume
optimization levels but it seems there were unresolved symbols in
iwlwifi after d9836fb4b9380e2ed1c38455fb31a3832b452671:
link_elf_obj: symbol iwl_mvm_send_roaming_forbidden_event undefined

Hide more of the currently unsupported (GPL-only) MEI (Management Engine)
code behind #ifdef to avoid this.

Reported by: dchagin
Tested by: dchagin
Sponsored by: The FreeBSD Foundation
MFC after: 3 days