mt76: connac: fix mt76_connac_gtk_rekey_tlv usage

The mistaken structure is introduced since we added the GTK rekey offload
to mt7663. The patch fixes mt76_connac_gtk_rekey_tlv structure according
to the MT7663 and MT7921 firmware we have submitted into
linux-firmware.git.

Fixes: b47e21e7 ("mt76: mt7615: add gtk rekey offload support")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Leon Yen <Leon.Yen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>