[css-mixins-1] Return value registration names are inconsistent in 'evaluate a custom function' algorithm

[anttijk]

6. If custom function has a return type, create a custom property registration with the name "return" (violating the usual rules for what a registration’s name can be), a syntax of the return type, an inherit flag of "false", and no initial value. Add the registration to registrations.

11. For each custom property registration of registrations except the registration with the name "result", set its initial value to the corresponding value in argument styles, set its syntax to the universal syntax definition, and prepend a custom property to body rule with the property name and value in argument styles.

Those should be consistent, likely "result" matching the descriptor name.