The internal key isn't actually in big-endian format so let's switch
to u128 which also happens to allow us to remove a sparse warning.

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