[net80211] add back in the ff/superg node init call that I accidentally

deleted.

It's mostly a no-op right now, so it shouldn't have changed behaviour.

sys/net80211/ieee80211_sta.c

@@ -1705,6 +1705,14 @@ sta_recv_mgmt(struct ieee80211_node *ni, struct mbuf *m0, int subtype,
 			ieee80211_ratectl_node_init(ni);
 		}
 
+		/*
+		 * Always initialise FF/superg state; we can use this
+		 * for doing A-MSDU encapsulation as well.
+		 */
+#ifdef	IEEE80211_SUPPORT_SUPERG
+		ieee80211_ff_node_init(ni);
+#endif
+
 		/*
 		 * Configure state now that we are associated.
 		 *