net: ethernet: aquantia: Using NETDEV_TX_OK instead 0.

Use NETDEV_TX_OK as the return value for successful transmission.

Signed-off-by: Pavel Belous <pavel.belous@aquantia.com>
Reviewed-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>