net/mlx5e: Block offload of outer header csum for GRE tunnel

The device is able to offload either the outer header csum or inner
header csum. The driver utilizes the inner csum offload. So, prohibit
setting of tx-gre-csum-segmentation and let it be: off[fixed].

Fixes: 27299841

 ("net/mlx5e: Support TSO and TX checksum offloads for GRE tunnels")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>