p54: fix off-by-one null pointer bug in p54_tx_80211

This patch fixes a regression which was introduced in:
"mac80211: move TX station pointer and restructure TX"

IP: p54_tx_80211+0x21/0x513 [p54common]
Oops: 0000 [#1] SMP
Modules linked in: p54usb p54common [...]
Pid: 13394, comm: hostapd 3.6.0-rc4-wl+
RIP: 0010:p54_tx_80211+0x21/0x513
RSP: 0018:...  EFLAGS: 00010292
[...]
Process hostapd
Stack:
[...]
Call Trace:
p54_bss_info_changed+0x204/0x21e [p54common]
ieee80211_del_station+0x16/0x32 [mac80211]
ieee80211_start_ap+0x10f/0x157 [mac80211]
nl80211_start_ap+0x315/0x361 [cfg80211]

p54_tx_80211 function is called as part of the
beacon update. The caller p54_bss_info_changed
has to supply a valid tx control struct, or
the control->sta will lead to a null pointer
dereference.
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/p54/main.c

@@ -139,6 +139,7 @@ static int p54_beacon_format_ie_tim(struct sk_buff *skb)
 static int p54_beacon_update(struct p54_common *priv,
 			struct ieee80211_vif *vif)
 {
+	struct ieee80211_tx_control control = { };
 	struct sk_buff *beacon;
 	int ret;
 
@@ -158,7 +159,7 @@ static int p54_beacon_update(struct p54_common *priv,
 	 * to cancel the old beacon template by hand, instead the firmware
 	 * will release the previous one through the feedback mechanism.
 	 */
-	p54_tx_80211(priv->hw, NULL, beacon);
+	p54_tx_80211(priv->hw, &control, beacon);
 	priv->tsf_high32 = 0;
 	priv->tsf_low32 = 0;