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

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

Warning(..//include/linux/mtd/nand.h:795): No description found for parameter 'ecc'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc:	David Woodhouse <dwmw2@infradead.org>
Cc:	Brian Norris <computersforpeace@gmail.com>
Cc:	linux-mtd@lists.infradead.org
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

include/linux/mtd/nand.h

@@ -766,6 +766,7 @@ struct nand_chip {
  * @options: stores various chip bit options
  * @id_len: The valid length of the @id.
  * @oobsize: OOB size
+ * @ecc: ECC correctability and step information from the datasheet.
  * @ecc.strength_ds: The ECC correctability from the datasheet, same as the
  *                   @ecc_strength_ds in nand_chip{}.
  * @ecc.step_ds: The ECC step required by the @ecc.strength_ds, same as the