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