common: image-fdt: correct fdt_blob for IMAGE_FORMAT_LEGACY

If condition of "(load == image_start || load == image_data)" is true,
should use "fdt_addr = load;", but not "fdt_blob = (char *)image_data;",
or fdt_blob will be overridden by "fdt_blob = map_sysmem(fdt_addr, 0);"
at the end of the switch case.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Max Krummenacher <max.krummenacher@toradex.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Suriyan Ramasami <suriyan.r@gmail.com>
Cc: Paul Kocialkowski <contact@paulk.fr>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>