devlink: Hold the instance lock in port_new / port_del callbacks

Let the core take the devlink instance lock around port_new and port_del
callbacks and remove the now redundant locking in the only driver that
currently use them.

Signed-off-by: Moshe Shemesh <moshe@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>