mac80211: reduce calculation costs of EWMA

This patch reduces the calculation costs of the EWMA macro from
"2x multiplication and 1 addition" down to "1x multiplication and
2x additions". This slightly improves performance depending on the
CPU architecture.

Signed-off-by: Thomas Huehn <[e-mail]>
Acked-by: Felix Fietkau <[e-mail]>
Signed-off-by: Johannes Berg <[e-mail]>

net/mac80211/rc80211_minstrel.h

···
 static inline int
 minstrel_ewma(int old, int new, int weight)
 {
-	return (new * (EWMA_DIV - weight) + old * weight) / EWMA_DIV;
+	int diff, incr;
+
+	diff = new - old;
+	incr = (EWMA_DIV - weight) * diff / EWMA_DIV;
+
+	return old + incr;
 }
 
 struct minstrel_rate_stats {