crypto: shash - remove useless crypto_yield() in shash_ahash_digest()

The crypto_yield() in shash_ahash_digest() occurs after the entire
digest operation already happened, so there's no real point.  Remove it.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>