mtd: spinand: Fill a default ECC provider/algorithm

The SPI-NAND layer default is on-die ECC because until now it was the
only one supported. New SPI-NAND chip flavors might use something else
as ECC engine provider but this will always be the default if the user
does not choose explicitly something else.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200930154109.3922-6-miquel.raynal@bootlin.com