smsc95xx: Support only IPv4 TCP/UDP csum offload

When TX checksum offload is used, if the computed checksum is 0 the
LAN95xx device do not alter the checksum to 0xffff.  In the case of ipv4
UDP checksum, it indicates to receiver that no checksum is calculated.
Under ipv6, UDP checksum yields a result of zero must be changed to
0xffff. Hence disabling checksum offload for ipv6 packets.

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

Reported-by: popcorn mix <popcornmix@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>