mt76: connac: move more mt7921/mt7915 mac shared code in connac lib

Move the following routines in mt76-connac lib since they are shared
between mt7915 and mt7921:
- mt76_connac2_tx_check_aggr
- mt76_connac2_txwi_free
- mt76_connac2_tx_token_put

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