mtd: nand: ecc: Add an I/O request tweaking mechanism

Currently, BCH and Hamming engine are sharing the same
tweaking/restoring I/O mechanism: they need the I/O request to fully
cover the main/OOB area. Let's make this code generic as sharing the
code between two drivers is already a win. Maybe other ECC engine
drivers will need it too.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200929230124.31491-2-miquel.raynal@bootlin.com