mtd: rawnand: Use the ECC framework user input parsing bits

Many helpers are generic to all NAND chips, they should not be
raw-NAND specific, so use the generic ones.

To avoid moving all the raw NAND core "history" into the generic NAND
layer, we keep a part of this parsing in the raw NAND core to ensure
backward compatibility.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200827085208.16276-20-miquel.raynal@bootlin.com