mac80211: minstrel, fix memory corruption

minstrel doesn't count max rate count in fact, since it doesn't use
a loop variable `i' and hence allocs space only for bitrates found in
the first band.

Fix it by involving the `i' as an index so that it traverses all the
bands now and finds the real max bitrate count.
Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/rc80211_minstrel.c

@@ -476,7 +476,7 @@ minstrel_alloc_sta(void *priv, struct ieee80211_sta *sta, gfp_t gfp)
 		return NULL;
 
 	for (i = 0; i < IEEE80211_NUM_BANDS; i++) {
-		sband = hw->wiphy->bands[hw->conf.channel->band];
+		sband = hw->wiphy->bands[i];
 		if (sband->n_bitrates > max_rates)
 			max_rates = sband->n_bitrates;
 	}