net: fec: Enable IP header hardware checksum

net: fec: Enable IP header hardware checksum

IP header checksum is calcalated by network layer in default.
To support software TSO, it is better to use HW calculate the
IP header checksum.

FEC hw checksum feature request the checksum field in frame
is zero, otherwise the calculative CRC is not correct.

For segmentated TCP packet, HW calculate the IP header checksum again,
it doesn't bring any impact. For SW TSO, HW calculated checksum bring
better performance.

Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>