[FIX] point_of_sale: Fix down/up rounding

While using the cash rounding, there were issues when rounding up and down.
The pre filled amount was correct but the rest of the transaction was wrong. (Wrong due, wrong change,...)
This commit re calculate the rounding applied when using up or down payment.

closes odoo/odoo#71219

Signed-off-by: pimodoo <pimodoo@users.noreply.github.com>