crypto: chacha20-generic - don't unnecessarily use atomic walk

chacha20-generic doesn't use SIMD instructions or otherwise disable
preemption, so passing atomic=true to skcipher_walk_virt() is
unnecessary.

Suggested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Martin Willi <martin@strongswan.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>