mtd: nand: correct comment on nand_chip badblockbits

The description for badblockbits is incorrect. I think someone just made
up a false description on the spot to satisfy some kerneldoc warning.

Signed-off-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>