[FIX] l10n_ch: Inconsistent return for `_find_or_create_bank_account()`

The method `_find_or_create_bank_account()` is defined for an
account.bank.statement.line so that it returns either the bank
account found or created. The extension made by l10n_ch does not
return the record found/created by a call to super(), which may
break reconciliations.

The code for the reconciliations in which a record is expected to
be returned can be seen at: https://github.com/odoo/odoo/blob/
c0eb0d41/addons/account/models/
account_bank_statement.py#L1232.

The original implementation of `_find_or_create_bank_account()`,
where a record is returned always, is at: https://github.com/


odoo/odoo/blob/c0eb0d41/addons/
account/models/account_bank_statement.py#L1282-L1291.

closes odoo/odoo#113147

Signed-off-by: Laurent Smet <las@odoo.com>