[FIX] mrp_subcontracting: Initial demand in a BO

Usecase to reproduce:
- Create a receipt with a quantity of 2
- Receive 1 and create a back order
- Receive 2 in the BO

It will only require the components for 2 units and not 3.

closes odoo/odoo#37305

Signed-off-by: Arnold Moyaux <amoyaux@users.noreply.github.com>