crypto: lib/chacha20poly1305 - use chacha20_crypt()

Use chacha20_crypt() instead of chacha_crypt(), since it's not really
appropriate for users of the ChaCha library API to be passing the number
of rounds as an argument.

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