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

L3 HW stats are implemented in mlxsw as RIF counters, and therefore the
code resides in spectrum_router. Exclude the offload xstats events from the
mlxsw_sp_netdevice_event_is_router() predicate, and instead recreate the
glue code in the router module.

Previously, the order of dispatch was that for events on tunnels, a
dedicated handler was called, which however did not handle HW stats events.
But there is nothing special about tunnel devices as far as HW stats: there
is a RIF associated with the tunnel netdevice, and that RIF is where the
counter should be installed. Therefore now, HW stats events are tested
first, independent of netdevice type. The upshot is that as of this commit,
mlxsw supports L3 HW stats work on GRE tunnels.

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>