mlxsw: spectrum: Use same trap group for IPv6 ND and ARP packets

Both packet types are needed for the same reason (neighbour discovery),
so associate them with the same trap group.

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