mtd: nand: ecc-bch: Cleanup and style fixes

Fix function headers, capitals and reword a little bit the comments
to make this driver more readable.

There is not functional change.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200929230124.31491-4-miquel.raynal@bootlin.com