crypto: ixp4xx - fallback when having more than one SG

Testing ixp4xx_crypto lead to:
alg: skcipher: ecb(des)-ixp4xx encryption overran dst buffer on test vector 0, cfg="two even aligned splits"

The HW overwrites destination always when sg_nents() > 1.
The problem seems that the HW always write areq->cryptlen bytes on the
last SG.
A comment in driver's code seems to give a clue that multiple SG was not
planned "This was never tested by Intel for more than one dst buffer, I think".
So let's add a fallback for this situation.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>