tools: ifdtool: Do not write region while its size is negative

We should ignore those regions whose size is negative. These are
typically optional and unused regions (like GbE and platform data).

Change-Id: I65ad01746144604a1dc0588b617af21f2722ebbf
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>