mtd: rawnand: Don't overwrite the error code from nand_set_ecc_soft_ops()

The error code received from nand_set_ecc_soft_ops() was overwritten,
drop this redundant assignment and use the error code received from
the callee.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200917075213.532161-4-tudor.ambarus@microchip.com