net80211: fix -Wunused-but-set-variable warnings

Put the offending variables under the appropriate #ifdefs
(mostly IEEE80211_DEBUG, in one case IEEE80211_SUPPORT_SUPERG, and
in two cases under notyet to revisit why these had been left
there but not used).

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 05ea7a3e5e748547a0229efa80be0e71eaf84f52)