net/mlx5e: Support partial GSO for tunnels over vlans

Offloading outer checksum on tunnels requires GSO partial, add it to
'vlan_features' to allow offloading tunnels over vlans.
For example, running GENEVE over vlan & ipv6 (mandatory UDP checksum)
now allows for hardware TSO instead of software segmentation in GSO
only.

Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Aya Levin <ayal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>