crypto: s390/sha512 - Fix sha512 state size

The sha512 state size in s390_sha_ctx is out by a factor of 8,
fix it so that it stays below HASH_MAX_DESCSIZE.  Also fix the
state init function which was writing garbage to the state.  Luckily
this is not actually used for anything other than export.

Reported-by: Harald Freudenberger <freude@linux.ibm.com>
Fixes: 572b5c46 ("crypto: s390/sha512 - Use API partial block handling")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>