crypto: x86/cast6-avx - convert to skcipher interface

Convert the AVX implementation of CAST6 from the (deprecated) ablkcipher
and blkcipher interfaces over to the skcipher interface.  Note that this
includes replacing the use of ablk_helper with crypto_simd.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>