nand: reinstate lazy bad block scanning

commit 2a8e0fc8 ("nand: Merge changes
from Linux nand driver") accidentally reverted commit
13f0fd94 ("NAND: Scan bad blocks
lazily.").

Reinstate the change, as amended by commit
ff49ea89 ("NAND: Mark the BBT as scanned
prior to calling scan_bbt.").

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