[FIX] pos_hr: save cashier correctly in db

Current behavior:
In PoS restaurant cashier were not saved correctly in db if you
switched table before making the payment of the order.

Steps to reproduce:
- Install PoS
- Activate restaurant on one of your PoS
- Activate Authorized employee
- Start a PoS session, and select a cashier
- Create an order on a table
- Leave the table and comeback to it
- Make the payment for the order
- Go in the PoS orders, the cashier wasn't saved

opw-2794574

closes odoo/odoo#88825

Signed-off-by: Masereel Pierre <pim@odoo.com>