mtd: nand: Add a helper to retrieve the number of ECC bytes per step

This operation is very common and deserves a helper. It of course only
works after the ECC engine initialization.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Tested-by: Adam Ford <aford173@gmail.com> #logicpd Torpedo
Link: https://lore.kernel.org/linux-mtd/20210127203020.9574-6-miquel.raynal@bootlin.com