mtd: rawnand: Try not to use the ECC private structures

Most of the time, there is no need to use the software ECC Hamming and
BCH algorithms private context to know their configuration. All the
data has been stored by their ->init_ctx() hook in the generic NAND
ECC engine structure, so use this one 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/20210127203020.9574-7-miquel.raynal@bootlin.com