qeth: use firmware MAC-address for layer2 hsi-devices

Real HiperSocket devices in layer2 mode have a firmware-created
MAC-address. This change enables the qeth driver to use this
firmware MAC-address for layer2 HiperSocket devices.

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>