devlink: Reduce struct devlink exposure

The declaration of struct devlink in general header provokes the
situation where internal fields can be accidentally used by the driver
authors. In order to reduce such possible situations, let's reduce the
namespace exposure of struct devlink.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>