crypto: sha256 - remove duplicate generic hash init function

crypto_sha256_init() and sha256_base_init() are the same repeated
implementations, remove the crypto_sha256_init() in generic
implementation, sha224 is the same process.

Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>