[FIX] l10n_ch: set field l10n_ch_currency_name to readonly

l10n_ch_currency_name is a related field to currency
Without this patch, when creating an invoice, the inverse of the
related field was computed, trying to write the name on the currency
(with the same value).
If a user did not have the required access rights (e.g. a user in the
group Billing can create an invoice but not write on res.currency by
default), he was unable to create the invoice.

Fixes odoo/odoo#31487
Closes odoo/odoo#31489

closes odoo/odoo#37373

X-original-commit: 5e49fa80
Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>