mtd: nand: Add a new helper to retrieve the ECC context

Introduce nand_to_ecc_ctx() which will allow to easily jump to the
private pointer of an ECC context given a NAND device. This is very
handy, from the prepare or finish ECC hook, to get the internal context
out of the NAND device object.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20211216111654.238086-14-miquel.raynal@bootlin.com