crypto: adiantum - propagate CRYPTO_ALG_ASYNC flag to instance

If the stream cipher implementation is asynchronous, then the Adiantum
instance must be flagged as asynchronous as well.  Otherwise someone
asking for a synchronous algorithm can get an asynchronous algorithm.

There are no asynchronous xchacha12 or xchacha20 implementations yet
which makes this largely a theoretical issue, but it should be fixed.

Fixes: 059c2a4d

 ("crypto: adiantum - add Adiantum support")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>