net: ipv6: mld: use proper MLDv2 modes

According to the RFC, MODE_IS_INCLUDE/MODE_IS_EXCLUDE codes should be
used while sendling Multicast Listener Reports in response to a query.
When performing a transition between states (e.g., when joining or
leaving a multicast group) CHANGE_TO_INCLUDE_MODE/CHANGE_TO_EXCLUDE_MODE
have to be used.

Signed-off-by: Konrad Derda <konrad.derda@nordicsemi.no>