rt2x00: Allow tx duplication for legacy rates in HT40 mode

Make use of the IEEE80211_TX_RC_DUP_DATA flag to duplicate a
transmission with legacy rates to both 20Mhz channels if set.
Also update the related comment in rt2800.h to describe the
behavior of the BW_40 flag for legacy rates.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/rt2x00/rt2x00ht.c b/drivers/net/wireless/rt2x00/rt2x00ht.c
index 48b6ed4..2023419 100644
--- a/drivers/net/wireless/rt2x00/rt2x00ht.c
+++ b/drivers/net/wireless/rt2x00/rt2x00ht.c
@@ -85,7 +85,13 @@
 		txdesc->rate_mode = RATE_MODE_HT_MIX;
 	if (txrate->flags & IEEE80211_TX_RC_GREEN_FIELD)
 		txdesc->rate_mode = RATE_MODE_HT_GREENFIELD;
-	if (txrate->flags & IEEE80211_TX_RC_40_MHZ_WIDTH)
+
+	/*
+	 * Set 40Mhz mode if necessary (for legacy rates this will
+	 * duplicate the frame to both channels).
+	 */
+	if (txrate->flags & IEEE80211_TX_RC_40_MHZ_WIDTH ||
+	    txrate->flags & IEEE80211_TX_RC_DUP_DATA)
 		__set_bit(ENTRY_TXD_HT_BW_40, &txdesc->flags);
 	if (txrate->flags & IEEE80211_TX_RC_SHORT_GI)
 		__set_bit(ENTRY_TXD_HT_SHORT_GI, &txdesc->flags);