net/mlx5: Make API mlx5_core_is_ecpf accept const pointer

Subsequent patch implements helper API which has mlx5_core_dev
as const pointer, make its caller API too const *.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Bodong Wang <bodong@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>