mlxsw: reg: Configure RIF to forward IPv6 multicast packets

Similarly to what was done in commit 4af5964e ("mlxsw: reg:
Configure RIF to forward IPv4 multicast packets by default") we now set
two additional bits to allow IPv6 multicast forwarding.

Signed-off-by: Yuval Mintz <yuvalm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>