cfg80211: implement cfg80211_get_station cfg80211 API

Implement and export the new cfg80211_get_station() API.
This utility can be used by other kernel modules to obtain
detailed information about a given wireless station.

It will be in particular useful to batman-adv which will
implement a wireless rate based metric.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

diff --cc include/net/cfg80211.h
Simple merge

diff --cc net/wireless/rdev-ops.h
Simple merge

diff --cc net/wireless/util.c
Simple merge