Fix the default parameter in print_list

Because sort_index = 0 by default, "if sort_index is not None" doesn't
make any sense.
Here sort_index should be None by default, which means "donot want to
sort".

Change-Id: I823ab75eb7a92bdc426dd96a3d1e56f187118729
Signed-off-by: Shane Wang <shane.wang@intel.com>