mtd: rawnand: omap: Use ECC information from the generic structures

As part of a previous fix, we imported the BCH internal structure in
order to get information about the BCH engine configuration.

It is best not to access private structure so instead, a small rework
has been done to export more information from the ECC engines. Now,
let's use these.

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/20210127203020.9574-8-miquel.raynal@bootlin.com