mt76: mt7663s: move tx/rx processing in the same txrx workqueue

Move mt7663 tx and rx processing in the same workqueue in order to
reduce jitter that can hit tcp performances. This is a preliminary patch
to switch to mt76 workers APIs

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