net: socionext: use new helper tcp_v6_gso_csum_prep

Use new helper tcp_v6_gso_csum_prep in additional network drivers.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>