minstrel: fix warning if lowest supported rate index is not 0

This patch fixes the following WARNING (caused by rix_to_ndx): "
>WARNING: at net/mac80211/rc80211_minstrel.c:69 minstrel_rate_init+0xd2/0x33a [mac80211]()
>[...]
>Call Trace:
> warn_on_slowpath+0x51/0x75
> _format_mac_addr+0x4c/0x88
> minstrel_rate_init+0xd2/0x33a [mac80211]
> print_mac+0x16/0x1b
> schedule_hrtimeout_range+0xdc/0x107
> ieee80211_add_station+0x158/0x1bd [mac80211]
> nl80211_new_station+0x1b3/0x20b [cfg80211]

The reason is that I'm experimenting with "g" only mode on a 802.11 b/g card.

Therefore rate_lowest_index returns 4 (= 6Mbit, instead of usual 0 = 1Mbit).
Since mi->r array is initialized with zeros in minstrel_alloc_sta,
rix_to_ndx has a hard time to find the 6Mbit entry and will trigged the WARNING.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/rc80211_minstrel.c

@@ -395,13 +395,15 @@ minstrel_rate_init(void *priv, struct ieee80211_supported_band *sband,
 {
 	struct minstrel_sta_info *mi = priv_sta;
 	struct minstrel_priv *mp = priv;
-	struct minstrel_rate *mr_ctl;
+	struct ieee80211_local *local = hw_to_local(mp->hw);
+	struct ieee80211_rate *ctl_rate;
 	unsigned int i, n = 0;
 	unsigned int t_slot = 9; /* FIXME: get real slot time */
 
 	mi->lowest_rix = rate_lowest_index(sband, sta);
-	mr_ctl = &mi->r[rix_to_ndx(mi, mi->lowest_rix)];
-	mi->sp_ack_dur = mr_ctl->ack_time;
+	ctl_rate = &sband->bitrates[mi->lowest_rix];
+	mi->sp_ack_dur = ieee80211_frame_duration(local, 10, ctl_rate->bitrate,
+				!!(ctl_rate->flags & IEEE80211_RATE_ERP_G), 1);
 
 	for (i = 0; i < sband->n_bitrates; i++) {
 		struct minstrel_rate *mr = &mi->r[n];
@@ -416,7 +418,7 @@ minstrel_rate_init(void *priv, struct ieee80211_supported_band *sband,
 
 		mr->rix = i;
 		mr->bitrate = sband->bitrates[i].bitrate / 5;
-		calc_rate_durations(mi, hw_to_local(mp->hw), mr,
+		calc_rate_durations(mi, local, mr,
 				&sband->bitrates[i]);
 
 		/* calculate maximum number of retransmissions before