mac80211: share STA information with driver

This patch changes mac80211 to share some more data about
stations with drivers. Should help iwlwifi and ath9k when
 they get around to updating, and might also help with
implementing rate control algorithms without internals.

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