mac80211: add helper for management / no-ack frame rate decision

All current rate control algorithms agree to send management and no-ack
frames at the lowest rate. They also agree to do this when sta
and the private rate control data is NULL. We add a hlper to mac80211
for this and simplify the rate control algorithm code.

Developers wishing to make enhancements to rate control algorithms
are for broadcast/multicast can opt to not use this in their
gate_rate() mac80211 callback.

Cc: Zhu Yi <yi.zhu@intel.com>
Acked-by: Reinette Chatre <reinette.chatre@intel.com>
Cc: ipw3945-devel@lists.sourceforge.net
Cc: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Cc: Derek Smithies <derek@indranet.co.nz>
Cc: Chittajit Mitra <Chittajit.Mitra@Atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/iwlwifi/iwl-agn-rs.c b/drivers/net/wireless/iwlwifi/iwl-agn-rs.c
index 3fea027..6328041 100644
--- a/drivers/net/wireless/iwlwifi/iwl-agn-rs.c
+++ b/drivers/net/wireless/iwlwifi/iwl-agn-rs.c
@@ -2481,13 +2481,8 @@
 	}
 
 	/* Send management frames and NO_ACK data using lowest rate. */
-	if (!ieee80211_is_data(hdr->frame_control) ||
-	    info->flags & IEEE80211_TX_CTL_NO_ACK || !sta || !lq_sta) {
-		info->control.rates[0].idx = rate_lowest_index(sband, sta);
-		if (info->flags & IEEE80211_TX_CTL_NO_ACK)
-			info->control.rates[0].count = 1;
+	if (rate_control_send_low(sta, priv_sta, txrc))
 		return;
-	}
 
 	rate_idx  = lq_sta->last_txrate_idx;