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>

net/wireless/rdev-ops.h

@@ -199,7 +199,7 @@ static inline int rdev_change_station(struct cfg80211_registered_device *rdev,
 }
 
 static inline int rdev_get_station(struct cfg80211_registered_device *rdev,
-				   struct net_device *dev, u8 *mac,
+				   struct net_device *dev, const u8 *mac,
 				   struct station_info *sinfo)
 {
 	int ret;