cfg80211: honor NL80211_RRF_NO_HT40{MINUS,PLUS}

Honor the NL80211_RRF_NO_HT40{MINUS,PLUS} flags in
reg_process_ht_flags_channel. Not doing so leads can lead
to a firmware assert in iwlwifi for example.

Fixes: b0d7aa59

 ("cfg80211: allow wiphy specific regdomain management")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>