Currently, "nand scrub" runs chip->scan_bbt at the end of
nand_erase_opts() even if NAND_SKIP_BBTSCAN flag is set.

It violates the intention of NAND_SKIP_BBTSCAN.

Move NAND_SKIP_BBTSCAN flag check to nand_block_checkbad() so that
chip->scan_bbt() is never run if NAND_SKIP_BBTSCAN is set.

Also, unset NAND_BBT_SCANNED flag instead of running chip->scan_bbt()
right after scrub.  We can be lazier here because the BBT is scanned
at the next call of nand_block_checkbad().

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