bootvx_fdt: fix missing 'fdt_fixup_ethernet(...)' on vxWorks boot

Before commit 26d61195 (fdt: Move fdt_fixup_ethernet to a common place)
the fdt_fixup_ethernet(...) was called during do_bootvx_fdt(...).

Afterwards the only (common) place for this fixup is during
image_setup_libfdt(...) and this is only called, at least on ARM
platform, from image_setup_linux(...).

All this ends up in the fact, that the fdt_fixup_ethernet(...) is only
called on booting a linux image and not on booting a vxWorks image.

We fix this with adding the fdt_fixup_ethernet(...) call again to
do_bootvx_fdt(...)

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>