mtd: nand: Mark the BBT as scanned prior to calling scan_bbt again

Commit 35c204d8 (nand: reinstate lazy bad block scanning)
broke NAND_BBT_USE_FLASH feature.

Its git-log claimed that it reinstated the change as by commit
fb49454b ("nand: reinstate lazy bad block scanning"), but it moved
"chip->options |= NAND_BBT_SCANNED" below "chip->scan_bbt(mtd);".

It causes recursion if scan_bbt does not find a flash based BBT
and tries to write one, and the attempt to erase the BBT area
causes a bad block check.

Reinstate commit ff49ea89 (NAND: Mark the BBT as scanned prior to
calling scan_bbt.).

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