team: Advertise tunneling offload features

When the underlying device supports offloads encapulated traffic,
we need to reflect that through the hw_enc_features field of the
team net-device.

This will cause the xmit path in the core networking stack to provide
team with encapsulated GSO frames to offload into the HW etc.

Using this over Mellanox ConnectX3-pro (mlx4 driver) card that supports
VXLAN offloads we got 36.0 Gbits/sec using eight iperf streams.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>