mtd: nand: reword nand_chip bad block interface comments

This remedies a few problems:

(1) The use of "the" vs. "a" is a little confusing, IMO.

(2) nand_chip.block_bad is used exclusively for checking the OOB bad
    block markers of a NAND. Any BBT functionality is handled in
    nand_bbt.c, so this description should differentiate itself from
    nand_bbt.c.

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>