pcnet: Do not receive external frames in loopback mode

While not explicitly stated in the spec, it was observed on real systems
that enabling loopback testing on the pcnet controller disables
reception of external frames. And some legacy software relies on it, so
provide this behavior.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>