fdt: fix setting MAC addresses for multiple interfaces

For multiple ethernet interfaces the FDT offset of '/aliases' will change as we
are adding MAC addresses to the FDT.
Therefore only the first interface ('ethernet0') will get properly updated in
the FDT, with the rest getting FDT errors when we try to set their MAC address.

Signed-off-by: Lev Iserovich <iserovil@deshawresearch.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>