crypto: x86/chacha - only unregister algorithms if registered

It's not valid to call crypto_unregister_skciphers() without a prior
call to crypto_register_skciphers().

Fixes: 84e03fa3 ("crypto: x86/chacha - expose SIMD ChaCha routine as library function")
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>