[FIX] purchase: Wrong quantity set up with packaging

Steps to reproduce the issue:

- Create a product P
- Enable packaging on Purchase
- Create a packaging PA such as 1 PA of P = 70 P
- Create a PO with P and set a quantity of 250 with PA as packaging

Bug:

A Warning was raised saying that we should purchase 280, PA was removed and a
quantity of 249.9 was set.

PS: PA was removed because the quantity was changed and function _onchange_suggest_packaging was triggered

opw:3007165

closes odoo/odoo#103070

Signed-off-by: William Henrotin (whe) <whe@odoo.com>