mtd: nand: sunxi: Fix ECC strength choice

When the requested ECC strength does not exactly match the strengths
supported by the ECC engine, the driver is selecting the closest
strength meeting the 'selected_strength > requested_strength'
constraint. Fix the fact that, in this particular case, ecc->strength
value was not updated to match the 'selected_strength'.

For instance, one can encounter this issue when no ECC requirement is
filled in the device tree while the NAND chip minimum requirement is not
a strength/step_size combo natively supported by the ECC engine.

Fixes: 1fef62c1 ("mtd: nand: add sunxi NAND flash controller support")
Cc: <stable@vger.kernel.org>
Suggested-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>