mt76: connac: move mt76_connac_tx_complete_skb in shared code

Since now txp structures are in common code we can reuse
mt76_connac_tx_complete_skb routine in mt7921e, mt7915e and mt7615e
drivers.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>