zigbee: Mark neighbor as not potential parent when full capacity detected

This merge fixes a bug in which potential parents are not discarded inside a device neighbor table when a router or coordinator is detected to have reached "full capacity" during a zigbee network join attempt (association).

This bug was reported by Piotr Biernacki in the ns-3 user groups. The potential fix was also suggested by him and it was used as the base for this MR.