cxgb4: Enable outer UDP checksum offload for T6

T6 adapters support outer UDP checksum offload for
encapsulated packets, hence enabling netdev feature flag
NETIF_F_GSO_UDP_TUNNEL_CSUM.

Signed-off-by: Arjun Vynipadath <arjun@chelsio.com>
Signed-off-by: Vishal Kulkarni <vishal@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>