net/mlx4_en: Don't allow aRFS for encapsulated packets

Driver doesn't support aRFS for encapsulated packets, return early error
in such a case.

Fixes: 1eb8c695 ("net/mlx4_en: Add accelerated RFS support")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>