Bluetooth fails to turn off with keyboard and headset connected

When HOGP keyboard is connecting and BT off is requested, if there
is any HOGP data that was received during this state, it causes the
following side-effects:
- LE data was not thrown away in disconnecting state, instead it is
now being used as indication of link establishement
-The above triggers the link open callback for fix channel and
causes the BT off failure.

Check L2CAP link state and always discard the fixed channel data
received in disconnecting state solves the problem.

Bug: 17793437
Change-Id: I633d5e6346e952daa2abd642effad2f260954fdf