mtd: nand: revive "nand scrub" command

Since commit ff94bc40 (mtd, ubi, ubifs: resync with Linux-3.14),
the "nand scrub" command has not been working.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Heiko Schocher <hs@denx.de>