mtd: rawnand: meson: fix OOB available bytes for ECC

It is incorrect to calculate number of OOB bytes for ECC engine using
some "already known" ECC step size (1024 bytes here). Number of such
bytes for ECC engine must be whole OOB except 2 bytes for bad block
marker, while proper ECC step size and strength will be selected by
ECC logic.

Fixes: 8fae856c

 ("mtd: rawnand: meson: add support for Amlogic NAND flash controller")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Arseniy Krasnov <AVKrasnov@sberdevices.ru>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230705065211.293500-1-AVKrasnov@sberdevices.ru