mt76: dma: introduce skb field in mt76_txwi_cache

Introduce skb field in mt76_txwi_cache. Moreover add txwi_flags
to mt76_driver_ops since new chipsets will release mt76_txwi_cache/skbs
at tx completion instead of dma one.
This is a preliminary patch to add mt7615 support

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