mtd: nand: ecc-bch: Use the public nsteps field

The software BCH ECC engine stores the nsteps variable in its own
private structure while it is also exported as a public ECC field.

Let's get rid of the redundant private one and let's use the
nand_ecc_context structure when possible.

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/20210127203020.9574-9-miquel.raynal@bootlin.com