batman-adv: mcast: fix mcast packet type counter on timeouted nodes

When a node which does not have the new batman-adv multicast packet type
capability vanishes then the according, global counter erroneously would
not be reduced in response on other nodes. Which in turn leads to the mesh
never switching back to sending with the new multicast packet type.

Fix this by reducing the according counter when such a node times out.

Fixes: 90039133 ("batman-adv: mcast: implement multicast packet generation")
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>