brcmsmac: use perimeter lock in add_interface() callback

All callbacks that access driver functions should do that under
perimeter lock protection. The add_interface() callback was lacking
this lock.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Piotr Haber <phaber@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/brcm80211/brcmsmac/mac80211_if.c

@@ -362,8 +362,10 @@ brcms_ops_add_interface(struct ieee80211_hw *hw, struct ieee80211_vif *vif)
 		return -EOPNOTSUPP;
 	}
 
+	spin_lock_bh(&wl->lock);
 	wl->mute_tx = false;
 	brcms_c_mute(wl->wlc, false);
+	spin_unlock_bh(&wl->lock);
 
 	return 0;
 }