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>

include/linux/mtd/nand.h

@@ -448,8 +448,9 @@ struct nand_buffers {
  *			will be copied to the appropriate nand_bbt_descr's.
  * @badblockpos:	[INTERN] position of the bad block marker in the oob
  *			area.
- * @badblockbits:	[INTERN] number of bits to left-shift the bad block
- *			number
+ * @badblockbits:	[INTERN] minimum number of set bits in a good block's
+ *			bad block marker position; i.e., BBM == 11110111b is
+ *			not bad when badblockbits == 7
  * @cellinfo:		[INTERN] MLC/multichip data from chip ident
  * @numchips:		[INTERN] number of physical chips
  * @chipsize:		[INTERN] the size of one chip for multichip arrays