[FIX] point_of_sale: cash rounding combination of cash and bank

There were two cases that caused the issue:
Consider a cash rounding with Precision 5 and only cash.

If you have an order that the total amount is 40 and pay 38 by
bank and 0 with cash (cash rounding), it will raise an error.

If you have an order that the total amount is 41 and pay 38 by
the bank and 5 with cash, it will raise another error.

opw-3302114

closes odoo/odoo#121626

Signed-off-by: Joseph Caburnay (jcb) <jcb@odoo.com>