mtd: nand: differentiate 1- vs. 2-byte writes when marking bad blocks

It seems that we have developed a bad-block-marking "feature" out of
pure laziness:

  "We write two bytes per location, so we dont have to mess with 16 bit
  access."

It's relatively simple to write a 1 byte at a time on x8 devices and 2
bytes at a time on x16 devices, so let's do it.

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>