mac80211: move TX station pointer and restructure TX

Remove the control.sta pointer from ieee80211_tx_info to free up
sufficient space in the TX skb control buffer for the upcoming
Transmit Power Control (TPC).
Instead, the pointer is now on the stack in a new control struct
that is passed as a function parameter to the drivers' tx method.

Signed-off-by: Thomas Huehn <thomas@net.t-labs.tu-berlin.de>
Signed-off-by: Alina Friedrichsen <x-alina@gmx.net>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
[reworded commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
diff --git a/drivers/net/wireless/rt2x00/rt2x00dev.c b/drivers/net/wireless/rt2x00/rt2x00dev.c
index a6b88bd..a59048f 100644
--- a/drivers/net/wireless/rt2x00/rt2x00dev.c
+++ b/drivers/net/wireless/rt2x00/rt2x00dev.c
@@ -194,7 +194,7 @@
 	 */
 	skb = ieee80211_get_buffered_bc(rt2x00dev->hw, vif);
 	while (skb) {
-		rt2x00mac_tx(rt2x00dev->hw, skb);
+		rt2x00mac_tx(rt2x00dev->hw, NULL, skb);
 		skb = ieee80211_get_buffered_bc(rt2x00dev->hw, vif);
 	}
 }