crypto: twofish - disable AVX2 implementation

It appears that the performance of 'vpgatherdd' is suboptimal for this kind of
workload (tested on Core i5-4570) and causes twofish_avx2 to be significantly
slower than twofish_avx. So disable the AVX2 implementation to avoid
performance regressions.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>