net/mlx5: Add helper functions for PTP callbacks

The PTP callback functions should not be used directly by internal
callers. Add helpers that can be used internally and externally.

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>
Reviewed-by: Mateusz Polchlopek <mateusz.polchlopek@intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>