MFC r325048:

Fix MAC address detection regression introduced by r324184

To accomodate all variaties of Pi DTS files floating around
we look for MAC address property either in DTS node for
USB ethernet (if it exists) or at predefined path
".../usb/hub/ethernet".

After r324184 smsc_fdt_find_eth_node started to return node
with compatibility string "usb424,ec00" as an eth node.
In imported GNU dts files this node still does not have
MAC address related property, and therefor following check for
"mac-address" and "local-mac-address" fails.

To make this logic more robust do not just search for the node
but also make sure it has required property, so if node with
accepted compatibility string exists but doesn't have the
property we fall back to looking for hardoded path mentioned above.