mac80211: minstrel_ht: improve sample rate selection

Always allow sampling of rates faster than the primary max throughput rate.
When the second max_tp_rate is higher than the first one, sample attempts were
previously skipped, potentially causing rate control to get stuck at a slightly
lower rate

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20210115120242.89616-7-nbd@nbd.name
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rc80211_minstrel_ht.c

@@ -1379,13 +1379,13 @@ minstrel_get_sample_rate(struct minstrel_priv *mp, struct minstrel_ht_sta *mi)
 	mrs = &mg->rates[sample_idx];
 	sample_idx += sample_group * MCS_GROUP_RATES;
 
-	/* Set tp_rate1, tp_rate2 to the highest / second highest max_tp_rate */
+	tp_rate1 = mi->max_tp_rate[0];
+
+	/* Set tp_rate2 to the second highest max_tp_rate */
 	if (minstrel_get_duration(mi->max_tp_rate[0]) >
 	    minstrel_get_duration(mi->max_tp_rate[1])) {
-		tp_rate1 = mi->max_tp_rate[1];
 		tp_rate2 = mi->max_tp_rate[0];
 	} else {
-		tp_rate1 = mi->max_tp_rate[0];
 		tp_rate2 = mi->max_tp_rate[1];
 	}