net/mlx5e: kTLS, Size of a Dump WQE is fixed

No Eth segment, so no dynamic inline headers.
The size of a Dump WQE is fixed, use constants and remove
unnecessary checks.

Fixes: d2ead1f3 ("net/mlx5e: Add kTLS TX HW offload support")
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Reviewed-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>