mac80211: use DECLARE_EWMA for ave_beacon_signal

It doesn't seem problematic to change the weight for the average
beacon signal from 3 to 4, so use DECLARE_EWMA. This also makes
the code easier to maintain since bugs like the one fixed in the
previous patch can't happen as easily.

With a fix from Avraham Stern to invert the sign since EMWA uses
unsigned values only.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/util.c

@@ -2543,7 +2543,7 @@ int ieee80211_ave_rssi(struct ieee80211_vif *vif)
 		/* non-managed type inferfaces */
 		return 0;
 	}
-	return ifmgd->ave_beacon_signal / 16;
+	return -ewma_beacon_signal_read(&ifmgd->ave_beacon_signal);
 }
 EXPORT_SYMBOL_GPL(ieee80211_ave_rssi);