ipip: convert to net_device_ops

Convert to network device ops. Needed to change to directly call
the init routine since two sides share same ops.  In the process
found by inspection a device ref count leak if register_netdevice failed.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>