devlink: make devlink_param_register/unregister static

There is no user outside the devlink code, so remove the export and make
the functions static. Move them before callers to avoid forward
declarations.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>