[FIX] point_of_sale: handle singleton exception for PoS config

Before this commit a singleton error is raised when writing to several
PoS configs at the same time.

After this commit, it is possible to write on multiple records at the
same time.

closes odoo/odoo#135586

Signed-off-by: Joseph Caburnay (jcb) <jcb@odoo.com>

addons/point_of_sale/models/pos_config.py

@@ -343,14 +343,14 @@ class PosConfig(models.Model):
     @api.constrains('limited_partners_amount', 'limited_partners_loading')
     def _check_limited_partners(self):
         for rec in self:
-            if rec.limited_partners_loading and not self.limited_partners_amount:
+            if rec.limited_partners_loading and not rec.limited_partners_amount:
                 raise ValidationError(
                     _("Number of partners loaded can not be 0"))
 
     @api.constrains('limited_products_amount', 'limited_products_loading')
     def _check_limited_products(self):
         for rec in self:
-            if rec.limited_products_loading and not self.limited_products_amount:
+            if rec.limited_products_loading and not rec.limited_products_amount:
                 raise ValidationError(
                     _("Number of product loaded can not be 0"))