intel/tools/error: Use do-while for field iterator loops.

intel/tools/error: Use do-while for field iterator loops.

while loops skip the first field of the instruction/structure, which
is not what the code intended.  It works out because the field we're
looking for doesn't happen to be first, but we ought to do it right
regardless.

Found while writing the next patch, where Kernel Start Pointer is
the first field of INTERFACE_DESCRIPTOR_DATA.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>