[FIX] hr_expense: allow saving expense with UOM readonly

Steps to repoduce:
- go to expense
- select Mileage [MIL]
- Save

Issue:
Canno save, different UOM

opw-3001568

closes odoo/odoo#102260

X-original-commit: 4621d776
Signed-off-by: Kevin Baptiste <kba@odoo.com>
Signed-off-by: Yolann Sabaux (yosa) <yosa@odoo.com>