sit: Use ipip6_tunnel_init as the ndo_init function.

ipip6_tunnel_init() sets the dev->iflink via a call to
ipip6_tunnel_bind_dev(). After that, register_netdevice()
sets dev->iflink = -1. So we loose the iflink configuration
for ipv6 tunnels. Fix this by using ipip6_tunnel_init() as the
ndo_init function. Then ipip6_tunnel_init() is called after
dev->iflink is set to -1 from register_netdevice().

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>