ftgmac100: Upgrade to NETIF_F_HW_CSUM

The documentation describes NETIF_F_IP_CSUM as deprecated
so let's switch to NETIF_F_HW_CSUM and use the helper to
handle unhandled protocols.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>