minstrel_ht: enable frame aggregation for fixed rate

When sample_idx is set to a value other than -1 it activates
the IEEE80211_TX_CTL_RATE_CTRL_PROBE flag which disables
frame aggregation. To allow frame aggregation during fixed
rate it is necessary to set max_tp_rate, max_tp_rate2 and
max_prob_rate instead of sample_idx.
Signed-off-by: Sylvain Roger Rieunier <sylvain.roger.rieunier@gmail.com>
[reword commit message a bit]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rc80211_minstrel_ht.c

@@ -626,8 +626,12 @@ minstrel_ht_get_rate(void *priv, struct ieee80211_sta *sta, void *priv_sta,
 
 #ifdef CONFIG_MAC80211_DEBUGFS
 	/* use fixed index if set */
-	if (mp->fixed_rate_idx != -1)
-		sample_idx = mp->fixed_rate_idx;
+	if (mp->fixed_rate_idx != -1) {
+		mi->max_tp_rate = mp->fixed_rate_idx;
+		mi->max_tp_rate2 = mp->fixed_rate_idx;
+		mi->max_prob_rate = mp->fixed_rate_idx;
+		sample_idx = -1;
+	}
 #endif
 
 	if (sample_idx >= 0) {