net/mlx5e: TC, Stub out ipv6 tun create header function

Improve mlx5e_route_lookup_ipv6 function structure by avoiding #ifdef then
return -EOPNOTSUPP in the middle of the function code.

To do so, we stub out mlx5e_tc_tun_create_header_ipv6 which is the only
caller of this helper function to avoid calling it altogether
when ipv6 is compiled out, which should also cleanup some compiler
warnings of unused variables.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Eli Cohen <eli@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>