[FIX] sale_stock: Correct incoterm when generating downpayment invoices

When creating an downpayment invoice for a SO with a incoterm different than the default one, this invoices ignores the incoterm you defined and uses the default value.

This issue is happening because in the _prepare_invoice_values is not passing the value of correct value of the incoterm and the default value is being used.

How to reproduce:
1. Active "incoterms" and choose a default incoterms in settings
2. Create a SO with a incoterm different than the default one
3. Create a downpayment invoice
4. In the new created invoice the incoterm is set as the default incoterm instead of the previously defined

closes odoo/odoo#120823

Opw: 3284556
Signed-off-by: Adrien Widart (awt) <awt@odoo.com>