mlx4_en: use NETIF_F_IP_CSUM | NETIF_F_IPV6_CSUM for tx csum at initialization

The former usage was to set the NETIF_F_HW_CSUM flag which is not used
in get_tx_csum. It caused Ethtool to show tx checksum as "on" even
though it was turned off in previous operation.

Signed-off-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>