mtd: fix kernel-doc warning in include/linux/mtd/nand.h

Fix kernel-doc warning in <linux/mtd/nand.h>:

Warning(include/linux/mtd/nand.h:659): No description found for parameter 'read_byte'

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Acked-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>