crypto: x86/chacha - Restore SSSE3 fallback path

The chacha_use_simd static branch is required for x86 machines that
lack SSSE3 support.  Restore it and the generic fallback code.

Reported-by: Eric Biggers <ebiggers@kernel.org>
Fixes: 9b440021 ("crypto: x86/chacha - Remove SIMD fallback path")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>