sit: allow to use rtnl ops on fb tunnel

rtnl ops where introduced by ba3e3f50 ("sit: advertise tunnel param via
rtnl"), but I forget to assign rtnl ops to fb tunnels.

Now that it is done, we must remove the explicit call to
unregister_netdevice_queue(), because  the fallback tunnel is added to the queue
in sit_destroy_tunnels() when checking rtnl_link_ops of all netdevices (this
is valid since commit 5e6700b3

 ("sit: add support of x-netns")).

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