Fix refunds for Card payment method

Typo during previous refactor lead to Card refunds failing. Not spotted
until now because the Card payment driver is only used for the backup
SumUp terminal at the Haymakers, and nowhere else.

quicktill/card.py

@@ -372,7 +372,7 @@ def start_payment(self, reg, transid, amount, outstanding):
                     ["You can't refund more than the amount due back."],
                     title="Refund too large")
                 return
-            _cardpopup(self, reg, transid, amount, refund=True)
+            _cardpopup(self.paytype.paytype, reg, transid, amount, refund=True)
             return
         if amount > outstanding:
             if self._cashback_method: