devlink: Create a helper function for one parameter registration

Create and use a helper function for one parameter registration.
Subsequent patch also will reuse this for driver facing routine to
register a single parameter.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>