ipv6: fix sparse warnings in privacy stable addresses generation

Those warnings reported by sparse endianness check (via kbuild test robot)
are harmless, nevertheless fix them up and make the code a little bit
easier to read.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 622c81d5 ("ipv6: generation of stable privacy addresses for link-local and autoconf")
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>