eth: sp7021: switch to netif_napi_add_tx()

The Tx NAPI should use netif_napi_add_tx().

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Wells Lu <wellslutw@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>