UPSTREAM: 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>

(cherry picked from commit 0b4b5f10)
Bug: 178411248
Change-Id: Ic260edcee45116ac1a9f03f2792fe577a2c629c6
Signed-off-by: Eric Biggers <ebiggers@google.com>