net: add change_carrier netdev op

This allows a driver to register change_carrier callback which will be
called whenever user will like to change carrier state. This is useful
for devices like dummy, gre, team and so on.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>