fix(donations): prevent warning on coupon filter

[miguelpeixe]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

The woocommerce_coupons_enabled filter is also applied in the Woo settings page. This PR prevents the donation-related filter from running in this context.

How to test the changes in this Pull Request:

1. While on the master branch, visit Woo settings page
2. Confirm you get the wc_get_product was called incorrectly. warning in the PHP logs
3. Check out this branch, refresh and confirm the warnings are gone

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[miguelpeixe]

This is not needed here and is being tackled in #2805