mlxsw: spectrum: Rename IPv6 ND trap group

The IPv6 Neighbour Discovery (ND) group will be used for various IPv6
packets, not all of which fall under the definition of ND, so rename it
to "IPV6" which is more appropriate.

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