tool: ifwitool: Fix buffer overflow

An incorrect 1st parameter is passed to the fix_member()
 function. Should use a pointer to the beginning of the parent structure
 (bpdt or subpart_dir, because are boxed), not to their fields. Otherwise,
 this leads to an overrun of the structure boundary, since in the
 fix_member() function, an 'offset' is made, relative to the 1st argument,
 which itself is an 'offset' from the beginning of the structure.

Signed-off-by: Mikhail Ilin <ilin.mikhail.ol@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>