net/mlx5: reject inner ethernet matching in GTP

The user is able to create a flow rule pattern with ETH after GTP
although it is not supported by the flex-parser configuration.

Failed the rule validation in such case with proper error message.

Fixes: 23c1d42c7138 ("net/mlx5: split flow validation to dedicated function")
Cc: stable@dpdk.org
Signed-off-by: Lior Margalit <lmargalit@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>