mtd: nand: ecc-bch: Stop exporting the private structure

The NAND BCH control structure has nothing to do outside of this
driver, all users of the nand_bch_init/free() functions just save it
to chip->ecc.priv so do it in this driver directly and return a
regular error code instead.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200929230124.31491-5-miquel.raynal@bootlin.com