Bluetooth: notify audio-manager when a2dp-sink disconnects

When an a2dp sink disconnects, is_src returns BT_STATUS_NOT_READY.
As the audio-manager is only informed when the return-value
is BT_STATUS_FAIL, this leads to an incorrect audio-routing
which still includes the disconnected device.
Fix this by treating BT_STATUS_NOT_READY the same as BT_STATUS_FAIL.

Change-Id: I4f1aafbda3481c4fc9b4c2f151fd4e042103f7c4