mt76: introduce mt76_tx_info data structure

Add mt76_tx_info as auxiliary data structure to pass values
to tx_prepare_skb pointer. This is a preliminary patch to add
support for new chipsets (e.g. mt7615)

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