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>

include/linux/mtd/nand.h

@@ -630,6 +630,7 @@ struct platform_device;
  *			ALE/CLE/nCE. Also used to write command and address
  * @write_buf:		platform specific function for write buffer
  * @read_buf:		platform specific function for read buffer
+ * @read_byte:		platform specific function to read one byte from chip
  * @priv:		private data to transport driver specific settings
  *
  * All fields are optional and depend on the hardware driver requirements