iwlwifi: find the correct first antenna

We can not assume antenna "A" is the first valid anttena for
all the NIC. Need to make sure choice the correct antenna based on
h/w configuration for transmit to avoid sending frame on invalid
antenna
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlwifi/iwl-agn-rs.c

@@ -760,6 +760,7 @@ static u32 rs_get_lower_rate(struct iwl_lq_sta *lq_sta,
 	u16 high_low;
 	u8 switch_to_legacy = 0;
 	u8 is_green = lq_sta->is_green;
+	struct iwl_priv *priv = lq_sta->drv;
 
 	/* check if we need to switch from HT to legacy rates.
 	 * assumption is that mandatory rates (1Mbps or 6Mbps)
@@ -773,7 +774,8 @@ static u32 rs_get_lower_rate(struct iwl_lq_sta *lq_sta,
 			tbl->lq_type = LQ_G;
 
 		if (num_of_ant(tbl->ant_type) > 1)
-			tbl->ant_type = ANT_A;/*FIXME:RS*/
+			tbl->ant_type =
+				first_antenna(priv->hw_params.valid_tx_ant);
 
 		tbl->is_ht40 = 0;
 		tbl->is_SGI = 0;