crypto: arm64/chacha20 - take may_use_simd() into account

To accommodate systems that disallow the use of kernel mode NEON in
some circumstances, take the return value of may_use_simd into
account when deciding whether to invoke the C fallback routine.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>