crypto: serpent - Fix sparse byte order warnings

This patch fixes the byte order markings in serpent.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Ard Biesheuvel <ardb@kernel.org> # arm64 big-endian
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>