[FIX] mrp_subcontracting: Updat quantity on confirmed PO

Steps to reproduce the bug:

- Let's consider a product P with a subcontracting BOM B and subcontractor S
- Create a purchase order PO with S as vendor
- Add P on PO and confirm it
- Change quantity of P two times

Bug:

A traceback was raised

opw:2419222

closes odoo/odoo#66440

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>