[fdt] Add one more heuristic to determine MAC address of the SMSC device

• If check for net,ethernet/usb,device compatible node fails, try to find .../usb/hub/ethernet, where ... is bus path that can depend on actual HW. net,ethernet/usb,device compatibity strings are FreeBSD custom invention that is used only in RPi DTBs and since there is no other way to tie USB device to FDT node we just do our best effort here to work with upstream device tree

• Use -1 value to indicate invalid phandle_t, 0 is valid phandle value and shouldn't be used as error signal