crypto: crypto4xx - replace crypto4xx_dev's scatter_buffer_size with constant

scatter_buffer_size is always set to PPC4XX_SD_BUFFER_SIZE.
I don't think there's any point in keeping the variable
around.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>