enic: Add vxlan offload support for IPv6 pkts

New adaptors supports vxlan offload for inner IPv6 and outer IPv6 vxlan
pkts.

Fw sets BIT(0) & BIT(1) in a1 if hw supports ipv6 inner & outer pkt
offload.

Signed-off-by: Govindarajulu Varadarajan <gvaradar@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>