net/mlx5: Add API to get mlx5_core_dev from mlx5_clock

The mdev is calculated directly from mlx5_clock, as it's one of the
fields in mlx5_core_dev. Move to a function so it can be easily
changed in next patch.

Signed-off-by: Jianbo Liu <jianbol@nvidia.com>
Reviewed-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>