devlink: devl_resource_register(): differentiate error codes

Differentiate error codes of devl_resource_register().

Replace one of -EINVAL exit paths by -EEXIST. This should aid developers
introducing new resources and registering them in the wrong order.

Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Link: https://patch.msgid.link/20241023131248.27192-4-przemyslaw.kitszel@intel.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>