crypto: sha-s390 - Reset index after processing partial block

The partial block handling in sha-s390 is broken when we get a
partial block that is followed by an update which fills it with
bytes left-over.  Instead of storing the newly left-over bytes
at the start of the buffer, it will be stored immediately after
the previous partial block.

This patch fixes this by resetting the index pointer.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>