mlxsw: spectrum_router: Only query neighbour activity when necessary

The driver periodically queries the device for activity of neighbour
entries in order to report it to the kernel's neighbour table.

Avoid unnecessary activity query when no neighbours are installed. Use
an atomic variable to track the number of neighbours, as it is read
without any locks.

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