mlxsw: spectrum_router: Embed netevent notifier block in router struct

We are going to need to respond to netevents notifying us about
multipath hash updates by configuring the device's hash parameters.

Embed the netevent notifier in the router struct so that we could
retrieve it upon notifications and use it to configure the device.

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