audio: always reset out written regardless of offload_state

Pcm offload use case relies on written count to return render position.
For pcm offload looping playback, if out written is not reset during
out_flush, the 2nd playback will get a wrong render position at the
beginning.

Change-Id: I61f5369458838d14e07aaf14ddb195c4d41d8409
CRs-Fixed: 2107254