darwin: fix resource leak (cached devices are never freed)

darwin: fix resource leak (cached devices are never freed)

For each new device (processed by hotplug code) darwin_ref_cached_device
is called twice - in process_new_device and in darwin_get_cached_device
(which is called from the first function). But when the device is detached
only one darwin_deref_cached_device is done - in darwin_destroy_device.
So there is inbalance between ref and deref which causes darwin_cached_devices
list to monotonically grow and consume more and more resources over time.
This commit fixes the problem by adding darwin_deref_cached_device call
to darwin_devices_detached.

Closes #52

Signed-off-by: Nathan Hjelm <hjelmn@mac.com>