mac80211: remove ieee80211_get_operstate()

ieee80211_get_operstate() was used by drivers in order to
know whether the sta link is up, but it's no longer needed
(nor used) as mac80211 notifies the drivers about
authorization changes (via the sta_state callback)

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>