Fix pointer overwrite in mt7921s_tx_prepare_skb and
mt7663_usb_sdio_tx_prepare_skb routines since in
commit '2a9e9857 ("mt76: fix possible pktid leak")
mt76_tx_status_skb_add() has been moved out of
mt7921s_write_txwi()/mt7663_usb_sdio_write_txwi() overwriting
hw key pointer in ieee80211_tx_info structure. Fix the issue saving
key pointer before running mt76_tx_status_skb_add().

Fixes: 2a9e9857 ("mt76: fix possible pktid leak")
Tested-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/eba40c84b6d114f618e2ae486cc6d0f2e9272cf9.1638193069.git.lorenzo@kernel.org