Remove extra fdt_fixup_ethernet() call

ft_cpu_setup() already calls fdt_fixup_ethernet(), calling it
in image_setup_libfdt() is both redundant and breaks any modifications
done by ft_board_setup(). Restore the old behavior by removing
the call in image_setup_libfdt()

Fixes: 13d06981 ("image: Add device tree setup to image library")
Signed-off-by: Joakim Tjernlund <joakim.tjernlund@infinera.com>