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>