net/mlx5e: Don't allow aRFS for encapsulated packets

Driver is yet to support aRFS for encapsulated packets, return early
error in such case.

Fixes: 18c908e4 ("net/mlx5e: Add accelerated RFS support")
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>