this fixes a bug when there are multiple mboard per device,
the occupancy int could be uninitialized to a bad value,
while subdev specs are being setup for the first time,
and cause allocation issues
Does away with the need for wax cast to get the tree.
You can still do this but it will eventually be removed.
There was some compiler issue on ubuntu 10.04 with any cast and a shared ptr to property tree.