mtd: rawnand: fsl_elbc: Make SW ECC work

Move the code that choses ECC into _attach_chip, which is executed only
after the chip->ecc.* properties were loaded from device-tree. This way
we know which ECC method was chosen by the device-tree and can set
methods appropriately.

The chip->ecc.*page methods should be set to fsl_elbc_*page only in HW
ECC mode.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>