net/mlx5e: Provide intermediate pointer to access IPsec struct

Improve readability by providing direct pointer to struct mlx5e_ipsec.

Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>