mtd: nand: use NAND_BBT_SCAN_MAXBLOCKS

In nand_bbt.c, a hardcoded value was used instead of the define meant
for that, so we use the define.

There's no functional change.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>