crypto: chacha_generic - remove unnecessary setkey() functions

Use chacha20_setkey() and chacha12_setkey() from
<crypto/internal/chacha.h> instead of defining them again in
chacha_generic.c.

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>