net: bcmgenet: Use netif_tx_napi_add() for TX NAPI

Before commit 7587935c ("net: bcmgenet: move NAPI initialization to
ring initialization") moved the code, this used to be
netif_tx_napi_add(), but we lost that small semantic change in the
process, restore that.

Fixes: 7587935c ("net: bcmgenet: move NAPI initialization to ring initialization")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>