mac80211: add support for mcs masks

* Handle MCS masks set by the user.
* Match rates provided by the rate control algorithm to the mask set,
  also in HT mode, and switch back to legacy mode if necessary.
* add debugfs files to observate the rate selection

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/include/net/mac80211.h b/include/net/mac80211.h
index 6501858..520eb4c 100644
--- a/include/net/mac80211.h
+++ b/include/net/mac80211.h
@@ -3551,6 +3551,7 @@
 	bool rts, short_preamble;
 	u8 max_rate_idx;
 	u32 rate_idx_mask;
+	u8 rate_idx_mcs_mask[IEEE80211_HT_MCS_MASK_LEN];
 	bool bss;
 };