net/mlx5e: Block offload of outer header csum for UDP tunnels

The device is able to offload either the outer header csum or inner
header csum. The driver utilizes the inner csum offload. Hence, block
setting of tx-udp_tnl-csum-segmentation and set it to off[fixed].

Fixes: b49663c8

 ("net/mlx5e: Add support for UDP tunnel segmentation with outer checksum offload")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>