[FIX] point_of_sale: Correctly compute refund cogs value

Current behavior:
When making a refund of an order in the PoS, the cost of good was
computed as if the refund was a classic order.

Steps to reproduce:
- Make sure you are in anglo-saxon accounting
- Create a category that use AVCO and automated valuation
- Create a product with a cost of 10€ that is part of the new category
- Create a PoS order with the product
- Refund the order
- Close the session, and check the accounting entries, the cost of good
  sold is 20€ instead of 0€

Note: this fix needs you to set the return picking type in the Operation
type of the PoS

opw-3149324

closes odoo/odoo#111252

Signed-off-by: William Henrotin (whe) <whe@odoo.com>