[FIX] point_of_sale: correctly link credit notes and invoice from PoS

Current behavior:
When creating a credit note from a PoS order, the credit note is not
linked to the invoice of the orginal order.

Steps to reproduce:
- Make an order in the PoS, validate it and invoice it
- Make a refund of this order and invoice it too.
- Go to the credit note, the original invoice is not mentionned in the
  reference field.

opw-3150637

closes odoo/odoo#119952

Signed-off-by: Engels Robin (roen) <roen@odoo.com>