mt76: switch to netif_napi_add_tx()

Switch to the new API not requiring passing in NAPI_POLL_WEIGHT.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220504163316.549648-3-kuba@kernel.org