Fix compatibility with WooCommerce Payment Gateway Roles

Fix compatibility with WooCommerce Payment Gateway Roles (and possibly others).

While working with WooCommerce Payment Gateway Roles it returned the MPM_Settings class instead of a valid gateway class. I modified the checks and MPM_Settings is only returned on the wc-settings page, otherwise the MPM_Gateway is returned.
Works perfectly, you can even assign individual roles for every enabled Mollie payment method.

Alvast bedankt! :+1: