mt76: add skb pointer to mt76_tx_info

Pass skb pointer to tx_prepare_skb through mt76_tx_info data structure.
This is a preliminary patch to properly support dma error path for
new chipsets (e.g. 7615)

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