mlxsw: spectrum_router: Fix build when IPv6 isn't enabled

When IPv6 isn't enabled the following error is generated:

ERROR: "nd_tbl" [drivers/net/ethernet/mellanox/mlxsw/mlxsw_spectrum.ko]
undefined!

Fix it by replacing 'arp_tbl' and 'nd_tbl' with 'tbl->family' wherever
possible and reference 'nd_tbl' only when IPV6 is enabled.

Fixes: d5eb89cf ("mlxsw: spectrum_router: Reflect IPv6 neighbours to the device")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>