wireless: fix regulatory HT40 allowed check

commit 1a919318 "regulatory: code cleanup"
changed is_ht40_allowed without considering that IEEE80211_CHAN_NO_HT40 is
not just one flag, but two.
This is causing HT40- to be blocked completely.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>