mlxsw: router: Enable neighbors to be created on stacked devices

Make the function mlxsw_router_neigh_construct search the rif according
to the neighbour dev other than the dev that was passed to the ndo, thus
allowing creating neigbhours upon stacked devices.

Fixes: 6cf3c971 ("mlxsw: spectrum_router: Add private neigh table")
Signed-off-by: Yotam Gigi <yotamg@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>