fix pos device create error

apps/pos_device/models.py

@@ -48,13 +48,13 @@ class Device(BaseModel):
     def __str__(self):
         return f'Device: {self.serial} - {self.id}'
 
-    def generate_device_identity(self):
+    def generate_device_identity(self):  # noqa
         """ generate identity for every device """
         prefix = "POS"
         while True:
             number_part = ''.join(random.choices(string.digits, k=6))
             code = f"{prefix}{number_part}"
-            if not self.objects.filter(short_code=code).exists():
+            if not Device.objects.filter(device_identity=code).exists():
                 return code
 
     def save(self, *args, **kwargs):