crypto: ahash - Stop legacy tfms from using the set_virt fallback path

Ensure that drivers that have not been converted to the ahash API
do not use the ahash_request_set_virt fallback path as they cannot
use the software fallback.

Reported-by: Eric Biggers <ebiggers@kernel.org>
Fixes: 9d7a0ab1 ("crypto: ahash - Handle partial blocks in API")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>