mlxsw: spectrum: Move netdevice NB to struct mlxsw_sp

So far, all netdevice notifications that the driver cared about were
related to its own ports, and mlxsw_sp could be retrieved from the
netdevice's private data. For IP-in-IP offloading however, the driver
cares about events on foreign netdevices, and getting at mlxsw_sp or
router data structures from the handler is inconvenient.

Therefore move the netdevice notifier blocks from global scope to struct
mlxsw_sp to allow retrieval from the notifier block pointer itself.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>