mtd: nand: sunxi: check ecc->size values

Verify that the ecc->size value is either 512 or 1024 bytes.
This should always be the case if this field was assigned to the
nand->ecc_step_size_ds value, but can be wrong when the user overloaded
this value with the nand-ecc-step-size DT property.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>