NAND: Properly create JFFS2 cleanmarkers.

As reported by Ilko Iliev <iliev@ronetix.at>, the "nand erase clean"
command is currently broken, and among other things causes all blocks
to be marked bad.

This implements it properly using MTD_OOB_AUTO, along with some
indentation fixes.

Signed-off-by: Scott Wood <scottwood@freescale.com>