[FIX] l10n_es_edi_sii: trim DescripcionOperacion for complience

Based on the SII documentation, DescripcionOperacion should have max 500
characters. Since it's based on invoice_origin field it can exceed this limit.

Now this value will be trimmed to the first 500 characters.

opw-3421094

closes odoo/odoo#130029

Signed-off-by: Florian Gilbert (flg) <flg@odoo.com>