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

Events involving VRF, as L3 concern, are handled in the router code, by the
helper mlxsw_sp_netdevice_vrf_event(). The handler is currently invoked
from the centralized dispatcher in spectrum.c. Instead, move the call to
the newly-introduced router-specific notifier handler.

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>