mtd: nand_ids: minor clean-ups

Clean-up the code a little bit:
  * clean-up commentaries.
  * move macro definitions to the top of the file.

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