mlxsw: spectrum_router: Set activity interval according to both neighbour tables

The neighbours' activity is currently dumped according to the ARP
table's DELAY_PROBE time, but with the introduction of IPv6 offload we
should set the interval according to the minimum between the ARP and
ndisc tables.

Signed-off-by: Arkadi Sharshvesky <arkadis@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>