mtd: atmel_nand: make ecc parameters same as definition

If the ecc parameter is not the same as definition, when the
mtd core check these parameters, it will give the error result.

Take the following as an example:

Calculate how many bits can be corrected in one page.
According to the ecc parameters definition,

one page correct bits = (mtd->writesize * ecc->strength) / ecc->size

take the following use case as an example:
mtd->writesize = 2048 bytes
ecc->strength = 4 bytes (for 512 bytes)

before this patch, the ecc->size = 2048, so the result is 4 bytes.
after this patch, the ecc->size = 512, so the result is 16 bytes.

So, align the ecc parameters the same as definition to correct
this kind of error.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Acked-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>