net/mlx5e: Protect against non-uplink representor for encap

TC encap offload is supported only for the physical uplink
representor. Fail for non uplink representor.

Fixes: 3e621b19 ("net/mlx5e: Support TC encapsulation offloads with upper devices")
Signed-off-by: Dmytro Linkin <dmitrolin@mellanox.com>
Reviewed-by: Eli Britstein <elibr@mellanox.com>
Reviewed-by: Vlad Buslov <vladbu@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>