drivers/net/oki-semi: Donot recompute IP header checksum

If I understand correct, NETIF_F_IP_CSUM only means the hardware
will compute the TCP/UDP checksum, IP checksum is always computed
in software

So as a workround of hardware unable to compute small packages
checksum, do not need to compute IP header checksum.

Signed-off-by: RongQing.Li <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>