ifwitool: Fix member access

On a second and third look, a recent patch seems to be writing to the
wrong place - updating offsets from the address of the pointer instead
of what the pointer points to.

Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: 2d1b2ac1 ("tool: ifwitool: Fix buffer overflow")
Acked-by: Sean Anderson <seanga2@gmail.com>