net/mlx5e: Use the inner headers to determine tc/pedit offload limitation on decap flows

In packets that need to be decaped the internal headers
have to be checked, not the external ones.

Fixes: bdd66ac0 ("net/mlx5e: Disallow TC offloading of unsupported match/action combinations")
Signed-off-by: Guy Shattah <sguy@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>