mlxsw: spectrum: Change ipv6 unregistered mc table

Point back the unregister IPv6 mc table to the bc table.
It is done since IPv6 mcast snooping is not supported for Spectrum yet.

Reported-by: Jiri Pirko <jiri@mellanox.com>
Fixes: 71c365bd ("mlxsw: spectrum: Separate bc and mc floods")
Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Signed-off-by: Yotam Gigi <yotamg@mellanox.com>
Tested-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>