NAND: Really ignore bad blocks when scrubbing

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Detlev Zundel <dzu@denx.de>
[scottwood@freescale.com: use chip instead of redundant priv_nand]
Signed-off-by: Scott Wood <scottwood@freescale.com>