[FIX] point_of_sale: correctly round total due in payment screen

Current behavior:
When you make a purchase that should be rounded down, for example 1.97
rounded down to 1.95. On the payment screen if you select a payment
method and pay with a greater amount than the base amount. The total due
would be rounded incorrectly to 2.00 instead of 1.95.

Steps to reproduce:
- Create a product with a price of 1.97
- Set a rouding down method of 0.05
- Open a pos session
- Add the product to the order
- Select a payment method, for example cash and enter an amount greater
  than the base amount, for example 5.00
- The total due will be 2.00 instead of 1.95

opw-3175726

closes odoo/odoo#113428

Signed-off-by: Heinz Robin (rhe) <rhe@odoo.com>