crypto: ixp4xx - Fix sparse warnings

This fixes a number of trivial sparse warnings in ixp4xx.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Tested-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>