net/mlx5: Add const to the key pointer of encryption key creation

Change key pointer to const void *, as there is no need to change the
key content. This is also to avoid modifying the key by mistake.

Signed-off-by: Jianbo Liu <jianbol@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>