Audio: Fix for audio loss on switching between speaker and USB headset

 - snd device reference count is not decremented when usb headset is
   unplugged, because touch tone is still active on usb headset.
   So when usb is plugged again device routing is not triggered as
   present and prev devices are same
 - Fix is to route audio to speaker as soon as usb headset is
   disconnected

   CRs-Fixed: 630425

Change-Id: Idfa37478fa19cd5a1c75c7e1d2f77b4dc02b311a