mac80211: Don't sample max throughput rate in minstrel_ht

The current max throughput rate is known to be good as otherwise it
wouldn't be the max throughput rate. Since rate sampling can introduce
some overhead (by adding RTS for example or due to not aggregating the
frame) don't sample the max throughput rate.
Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/rc80211_minstrel_ht.c

@@ -567,6 +567,13 @@ minstrel_get_sample_rate(struct minstrel_priv *mp, struct minstrel_ht_sta *mi)
 	sample_idx += mi->sample_group * MCS_GROUP_RATES;
 	minstrel_next_sample_idx(mi);
 
+	/*
+	 * Sampling might add some overhead (RTS, no aggregation)
+	 * to the frame. Hence, don't use sampling for the currently
+	 * used max TP rate.
+	 */
+	if (sample_idx == mi->max_tp_rate)
+		return -1;
 	/*
 	 * When not using MRR, do not sample if the probability is already
 	 * higher than 95% to avoid wasting airtime