net/mlx5: Rework devlink port alloc/free into init/cleanup

In order to prepare the devlink port registration function to be common
for PFs/VFs and SFs, change the existing devlink port allocation and
free functions into PF/VF init and cleanup, so similar helpers could be
later on introduced for SFs. Make the init/cleanup helpers responsible
for setting/clearing the vport->dl_port pointer.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Shay Drory <shayd@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>