mt76: mt7915: use ieee80211_free_txskb to free tx skbs

Using dev_kfree_skb for tx skbs breaks AQL. This worked until now only
by accident, because a mac80211 issue breaks AQL on drivers with firmware
rate control that report the rate via ieee80211_tx_status_ext as struct
rate_info.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200812144943.91974-1-nbd@nbd.name