mac80211: fix ethtool stats for non-station interfaces

As reported in https://bugzilla.kernel.org/show_bug.cgi?id=60514,
the station loop never initialises 'sinfo' and therefore adds up
a stack values, leaking stack information (the number of times it
adds values is easily obtained another way.)

Fix this by initialising the sinfo for each station to add.

Cc: stable@vger.kernel.org
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>

net/mac80211/cfg.c

@@ -666,6 +666,8 @@ static void ieee80211_get_et_stats(struct wiphy *wiphy,
 			if (sta->sdata->dev != dev)
 				continue;
 
+			sinfo.filled = 0;
+			sta_set_sinfo(sta, &sinfo);
 			i = 0;
 			ADD_STA_STATS(sta);
 		}