brcm80211: fix null pointer access

Do not unconditionally access the chan variable in
brcmf_cfg80211_mgmt_tx() as it may be NULL.
Use freq instead.

Introduced by c2ff8cad64233b539c71a27e2a6e324001143ef0
("brcm80211: make mgmt_tx in brcmfmac accept a NULL channel")

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/brcm80211/brcmfmac/wl_cfg80211.c

@@ -4050,8 +4050,7 @@ brcmf_cfg80211_mgmt_tx(struct wiphy *wiphy, struct wireless_dev *wdev,
 		       le16_to_cpu(action_frame->len));
 
 		brcmf_dbg(TRACE, "Action frame, cookie=%lld, len=%d, freq=%d\n",
-			  *cookie, le16_to_cpu(action_frame->len),
-			  chan->center_freq);
+			  *cookie, le16_to_cpu(action_frame->len), freq);
 
 		ack = brcmf_p2p_send_action_frame(cfg, cfg_to_ndev(cfg),
 						  af_params);