mtd: rawnand: Get rid of chip->ecc.priv

nand_ecc_ctrl embeds a private pointer which only has a meaning in the
sunxi driver. This structure will soon be deprecated, but as this
field is actually not needed, let's just drop it.

Cc: Maxime Ripard <mripard@kernel.org>
Cc: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20200929230124.31491-11-miquel.raynal@bootlin.com