net: airoha: Simplify Tx napi logic

Simplify Tx napi logic relying just on the packet index provided by
completion queue indicating the completed packet that can be removed
from the Tx DMA ring.
This is a preliminary patch to add Qdisc offload for airoha_eth driver.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20241029-airoha-en7581-tx-napi-work-v1-2-96ad1686b946@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>