[FIX] stock: save correctly the selected SN in stock move

Steps to reproduce the bug:
- Create a product and enable serial number option
- Create one “on hand qty” with SN e.g: “001”
- Create a SO for SN product
- Confirm SO and check the delivery order
- In the Operations tab > Enable Serial Number field
- “001” SN is already reserved in Detailed Operations and the qty done is 0 but the Serial Number field in “stock.move” is empty
- Edit and add “001” in the SN field
- save

Problem:
- The SN is deleted and the quantity done in the `”stock.move.line”` is not updated

Solution:
Update the quantity done in the `“stock.move.line”` linked to the selected SN

opw-2623101

closes odoo/odoo#77521

Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>