mlxsw: spectrum: Use per-netns netdevice notifier registration

The mlxsw_sp instance is not interested in events happening in other
network namespaces. So use "_net" variants for netdevice notifier
registration/unregistration and get only events which are happening in
the net the instance is in.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>