wifi: improve fix to avoid device double release on disable

wifi: improve fix to 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 improves upon and revises the prior patch to address this
issue by recognizing that hidden networks involve a slightly different
code path and timing and that both this and the prior case can be
addressed by tracking the device scanning state and only releasing the
device when the state is asserted.