Fix P2P crash when wifi device is unmanaged
See https://github.com/NetworkManager/NetworkManager/pull/299#issuecomment-464999486
There were two separate issues:
- The P2P device would not be destroyed together with its parent NMDeviceWifi
- The P2P device could switch into the
DISCONNECTED
state even after the management supplicant interface was unset. We need to force the state into DISABLED when the NMDeviceWifiP2P is waiting for the supplicant interface.