mac80211: re-add HT disabling

The IEEE80211_STA_TKIP_WEP_USED flag is used internally to
disable HT when WEP or TKIP are used. Now that cfg80211 is
giving us the required information, we can set the flag
appropriately again.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>