mlxsw: spectrum_router: Avoid unnecessary neighbour updates

Avoid updating neighbour and adjacency entries in hardware when the
neighbour is already connected and its MAC address did not change. This
can happen, for example, when neighbour transitions between valid states
such as 'NUD_REACHABLE' and 'NUD_DELAY'.

This is especially important for resilient hashing as these updates will
result in adjacency entries being marked as active.

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