serial: return a return code in reset_card()

The method reset_card does not return a return code, while the
coresponding pcsc implementation does return 1 on success.

Change-Id: I658dd6857580652696b4a77e7d6cfe5778f09eff

pySim/transport/serial.py

@@ -104,6 +104,7 @@ class SerialSimLink(LinkBase):
             raise NoCardError()
         elif rv < 0:
             raise ProtocolError()
+        return rv
 
     def _reset_card(self):
         self._atr = None