Steps to reproduce the bug:
- Create a company C with currency = USD
- Create a vendor V with a supplier currency = EUR
- Create a PO for V with one line, the default currency is EUR
- Save

Bug:

The currency of the PO was USD when saving because the field currency is
in readonly when the state is draft.

opw:2035640

closes odoo/odoo#35144

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>