crypto: blake2b - merge blake2 init to api callback

The call chain from blake2b_init can be simplified because the param
block is effectively zeros, besides the key.

- blake2b_init0 zeroes state and sets IV
- blake2b_init sets up param block with defaults (key and some 1s)
- init with key, write it to the input buffer and recalculate state

So the compact way is to zero out the state and initialize index 0 of
the state directly with the non-zero values and the key.

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