According to RFC 2460, IPv6 UDP calculated checksum yields a result

of zero must be changed to 0xffff, however this feature is not
supported by smsc95xx family hence enable csum offload only for
IPv4 TCP/UDP packets.

Signed-off-by: Nisar Sayed <Nisar.Sayed@microchip.com>

Reported-by: popcorn mix <popcornmix@gmail.com>