docs: mtd: Avoid htmldocs warnings

Moving files around produced the following warnings:

       Error: Cannot open file drivers/mtd/nand/raw/nand_ecc.c
       Error: Cannot open file drivers/mtd/nand/raw/nand_ecc.c

Fix one by just dropping the reference because it is not relevant, the
other by using a better noun instead of a file name.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201113123831.32429-1-miquel.raynal@bootlin.com