sit: always notify change when params are updated

netdev_state_change() was called only when end points or link was updated. Now
that all parameters are advertised via netlink, we must advertise any change.

This patch also prepares the support of sit tunnels management via rtnl. The
code which update tunnels will be put in a new function.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>