mvneta: Driver and hardware supports IPv6 offload, so enable it

mvneta: Driver and hardware supports IPv6 offload, so enable it

The mvneta driver and hardware supports IPv6 offload, however it
isn't enabled. Set the NETIF_F_IPV6_CSUM feature to inform the
network layer that this driver can offload IPV6 TCP and UDP
checksums. This change has been tested on an Armada 370 and the
feature support confirmed with several device datasheets
including the Armada XP and Armada 3700.

Signed-off-by: Andrew Pilloud <andrewpilloud@igneoussystems.com>
Signed-off-by: David S. Miller <davem@davemloft.net>