mt76: remove obsolete .add_buf() from struct mt76_queue_ops

		     int idx, int n_desc, int bufsize,

		     u32 ring_base);

	int (*add_buf)(struct mt76_dev *dev, struct mt76_queue *q,

		       struct mt76_queue_buf *buf, int nbufs, u32 info,

		       struct sk_buff *skb, void *txwi);

	int (*tx_queue_skb)(struct mt76_dev *dev, enum mt76_txq_id qid,

			    struct sk_buff *skb, struct mt76_wcid *wcid,

			    struct ieee80211_sta *sta);