mtd: rawnand: Introduce nand_set_ecc_on_host_ops()

There were too many levels of indentation and the code was
hard to read. Introduce a new function, similar to
nand_set_ecc_soft_ops().

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/20200917075213.532161-3-tudor.ambarus@microchip.com