wifi: avoid device double release on disable

wifi: avoid device double release on disable

Per MEEGO-25999 <https://bugs.meego.com/show_bug.cgi?id=25999>, there
exists a race between scan_callback and wifi_disable such that a care-
fully-timed deassertion of the technology Powered property between
throw_wifi_scan and scan_callback will lead to a device reference
count underflow. A second Powered property false -> true -> false
transition will then deallocate the device and Wi-Fi will no longer
function until connman or the system is restarted.

This patch addresses that race by tracking and observing the disabling
state such that scan_callback ad wifi_disable can effectively negotiate
which has performed the device release, thereby avoiding the double
release and resulting underflow.