crypto: arm/ghash - fix big-endian bug in ghash

This fixes a bug in the new v8 Crypto Extensions GHASH code
that only manifests itself in big-endian mode.

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