mlxsw: spectrum: Move handling of router events to router code

The events NETDEV_PRE_CHANGEADDR, NETDEV_CHANGEADDR and NETDEV_CHANGEMTU
have implications for in-ASIC router interface objects, and as such are
handled in the router module. Move the handling from the central dispatcher
in spectrum.c to the new notifier handler in the router module.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>