crypto: blake2b - merge _update to api callback

Now that there's only one call to blake2b_update, we can merge it to the
callback and simplify. The empty input check is split and the rest of
code un-indented.

Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>