mwl8k: Fixing sta dereference when ieee80211_tx_info->control.sta is NULL

Following oops was seen on SMP machine

>BUG: unable to handle kernel NULL pointer dereference at 00000012
>IP: [<f8c56691>] mwl8k_tx+0x20e/0x561 [mwl8k]
>*pde = 00000000
>Oops: 0000 [#1] SMP
>Modules linked in: mwl8k mac80211 cfg80211 [last unloaded: cfg80211]

As ieee80211_tx_info->control.sta may be NULL during ->tx call, avoiding sta
dereference in such scenario with the following patch.
Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/mwl8k.c

@@ -1891,9 +1891,9 @@ mwl8k_txq_xmit(struct ieee80211_hw *hw, int index, struct sk_buff *skb)
 
 	txpriority = index;
 
-	if (ieee80211_is_data_qos(wh->frame_control) &&
-	    skb->protocol != cpu_to_be16(ETH_P_PAE) &&
-	    sta->ht_cap.ht_supported && priv->ap_fw) {
+	if (priv->ap_fw && sta && sta->ht_cap.ht_supported
+			&& skb->protocol != cpu_to_be16(ETH_P_PAE)
+			&& ieee80211_is_data_qos(wh->frame_control)) {
 		tid = qos & 0xf;
 		mwl8k_tx_count_packet(sta, tid);
 		spin_lock(&priv->stream_lock);