mt76: move token_lock, token and token_count in mt76_dev

Move token_lock, token and token_count data structures in mt76_dev.
This is a preliminary patch to move token management in mt76 common
module since it is shared by mt7615, mt7915 and mt7921 drivers.

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