mac80211: insert AP sta entry after filling it

We never clearly defined the semantics of the sta_notify callback
and it was originally posted for iwlwifi which still doesn't use
it at all. With the recent HT rework ath9k started relying on it,
but I made a mistake there in that I made ath9k assume the HT
information has already been filled in at sta_notify time. This
isn't a hard thing to do, so do it.

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