[FIX] mrp: write with multiple workorders in self

This PR https://github.com/odoo/odoo/pull/97805

 wrongly assumed that
self contained a single workorder. This would raise an error if there
were multiple records in self.

Solution:
Modify calls on `self` to `workorder` (as we already iterate through
them)

closes odoo/odoo#101580

Signed-off-by: William Henrotin (whe) <whe@odoo.com>