net/eth/ibmveth: Fixup retrieval of MAC address

Some ancient pHyp versions used to create a 8 bytes local-mac-address
property in the device-tree instead of a 6 bytes one for veth.

The Linux driver code to deal with that is an insane hack which also
happens to break with some choices of MAC addresses in qemu by testing
for a bit in the address rather than just looking at the size of the
property.

Sanitize this by doing the latter instead.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: <stable@vger.kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>