devlink: Avoid overwriting port attributes of registered port

Cited commit in fixes tag overwrites the port attributes for the
registered port.

Avoid such error by checking registered flag before setting attributes.

Fixes: 71ad8d55

 ("devlink: Replace devlink_port_attrs_set parameters with a struct")
Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20201111034744.35554-1-parav@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>