[FIX] point_of_sale: respect setting only_round_cash_method

STEPS:
* configure rounding:
  - cash_rounding=True
  - only_round_cash_method=True
  - round method: 1.0 Down

* open a session
* add products with subtotal in decimals (such as 6.35)
* select Bank payment method and process the order

BEFORE: error "You have to round your payments lines"

AFTER: order is processed and marked as paid

---

https://github.com/odoo/odoo/commit/42425996e36d9cb1fc1391402cf6d00df4aea019


opw-2426027

closes odoo/odoo#64270

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