nand: reinstate lazy bad block scanning

Commit ff94bc40
("mtd, ubi, ubifs: resync with Linux-3.14")
accidentally reverted part of the commit
13f0fd94
("NAND: Scan bad blocks lazily.").

Reinstate the change as by commit
fb49454b
("nand: reinstate lazy bad block scanning")

Signed-off-by: Rostislav Lisovy <lisovy@merica.cz>
Acked-by: Heiko Schocher <hs@denx.de>