mac80211: optimize minstrel_ewma

Use powers of two in ewma of minstrel.
This changes :
- EWMA_DIV   from 100 to 2^7
- EWMA_LEVEL from 75 (/EWMA_DIV=100) to 2^6 + 2^5 (/EWMA_DIV=128)

Note that this changes EWMA_DIV - EWMA_LEVEL from 25 to 2^5 and keeps
EWMA_LEVEL / EWMA_DIV == 0.75.

Signed-off-by: Karl Beldan <karl.beldan@rivierawaves.com>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rc80211_minstrel.h

@@ -9,7 +9,8 @@
 #ifndef __RC_MINSTREL_H
 #define __RC_MINSTREL_H
 
-#define EWMA_LEVEL	75	/* ewma weighting factor [%] */
+#define EWMA_LEVEL	96	/* ewma weighting factor [/EWMA_DIV] */
+#define EWMA_DIV	128
 #define SAMPLE_COLUMNS	10	/* number of columns in sample table */
 
 
@@ -27,7 +28,7 @@
 static inline int
 minstrel_ewma(int old, int new, int weight)
 {
-	return (new * (100 - weight) + old * weight) / 100;
+	return (new * (EWMA_DIV - weight) + old * weight) / EWMA_DIV;
 }