net: ipv6: minor: use in6addr_any in token init

Since we check for !ipv6_addr_any(&in6_dev->token) in
addrconf_prefix_rcv(), make the token initialization on
device setup more intuitive by using in6addr_any as an
initializer.

Suggested-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>