mtd: nand: rename the id field of 'struct nand_flash_dev'

mtd: nand: rename the id field of 'struct nand_flash_dev'

The 'id' is a bit confusing name because NAND IDs are multi-byte. Re-name
it to 'dev_id' to make it clear that this is the "device ID" part (the second
byte).

While on it, clean-up the commentary for 'struct nand_flash_dev'.

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