minstrel_ht: fixed rate mode through debugfs

Found several threads about fixed rate mode in minstrel_ht for test
environments, but no patches for it.

This patch provides such a mode through debugfs.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/net/mac80211/rc80211_minstrel_ht.c b/net/mac80211/rc80211_minstrel_ht.c
index 333b511..66a1eeb 100644
--- a/net/mac80211/rc80211_minstrel_ht.c
+++ b/net/mac80211/rc80211_minstrel_ht.c
@@ -609,6 +609,13 @@
 
 	info->flags |= mi->tx_flags;
 	sample_idx = minstrel_get_sample_rate(mp, mi);
+
+#ifdef CONFIG_MAC80211_DEBUGFS
+	/* use fixed index if set */
+	if (mp->fixed_rate_idx != -1)
+		sample_idx = mp->fixed_rate_idx;
+#endif
+
 	if (sample_idx >= 0) {
 		sample = true;
 		minstrel_ht_set_rate(mp, mi, &ar[0], sample_idx,