crypto: s390/sha512 - Initialise upper counter to zero for sha384

Initialise the high bit counter to zero in sha384_init.

Also change the state initialisation to use ctx->sha512.state
instead of ctx->state for consistency.

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