net/mlx5: Fix offset of tisc bits reserved field

First reserved field is off by one instead of reserved_at_1 it should be
reserved_at_2, fix that.

Fixes: a12ff35e

 ("net/mlx5: Introduce TLS TX offload hardware bits and structures")
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>