Avoid duplicate disconnection callbacks for fixed channel connections

Fixed channel connections are getting a disconnect callback when the
HCI command is issued and when the HCI disconnect completes. This
causes problems if the upper layer trying to reconnect at link
disconnecting state.
Triggering the disconnect callback only once solves this problem.

Original author: Chaojing Sun <cjsun@broadcom.com>

Bug: 19816438
Change-Id: Ib661c968e586975a7fc7244e2d0745f71d52e3e9