tools: mtk_image: split gfh header verification into a new function

The verification code of gfh header for NAND and non-NAND are identical.
It's better to define a individual function to reduce redundancy.

Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>