ath9k: fix a tx rate duration calculation bug

The rate pointer variable for a rate series is used in a loop before it is
initialized. This went unnoticed because it was used earlier for the RTS/CTS
rate. This bug can lead to the wrong PHY type being passed to the
duration calculation function.
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath9k/xmit.c

@@ -1001,13 +1001,13 @@ static void ath_buf_set_rate(struct ath_softc *sc, struct ath_buf *bf,
 		}
 
 		/* legacy rates */
+		rate = &sc->sbands[tx_info->band].bitrates[rates[i].idx];
 		if ((tx_info->band == IEEE80211_BAND_2GHZ) &&
 		    !(rate->flags & IEEE80211_RATE_ERP_G))
 			phy = WLAN_RC_PHY_CCK;
 		else
 			phy = WLAN_RC_PHY_OFDM;
 
-		rate = &sc->sbands[tx_info->band].bitrates[rates[i].idx];
 		info->rates[i].Rate = rate->hw_value;
 		if (rate->hw_value_short) {
 			if (rates[i].flags & IEEE80211_TX_RC_USE_SHORT_PREAMBLE)