staging: brcm80211: bugfix for control.sta NULL ptr dereference

Mac80211 can transmit packets where the control.sta field is NULL.
The driver dereferenced this. Bugfix was to only dereference a non NULL
ieee80211_sta pointer.
Signed-off-by: NRoland Vossen <rvossen@broadcom.com>
Reviewed-by: NArend van Spriel <arend@broadcom.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/staging/brcm80211/brcmsmac/wlc_main.c

@@ -6613,6 +6613,7 @@ wlc_dotxstatus(struct wlc_info *wlc, tx_status_t *txs, u32 frm_tx2)
 	tx_info = IEEE80211_SKB_CB(p);
 	h = (struct ieee80211_hdr *)((u8 *) (txh + 1) + D11_PHY_HDR_LEN);
 
+	if (tx_info->control.sta)
 		scb = (struct scb *)tx_info->control.sta->drv_priv;
 
 	if (N_ENAB(wlc->pub)) {