msm: audio: qdsp6v2: Fix seek issue in tunnel player

- Seeking to 0 position during tunnel playback, music stream
  jumps to incorrect postion.
- Initial buffer after seek to new position is corrupted as
  flush ioctl is not synchronous. Ioctl FLUSH is posting
  CMD_FLUSH to DSP, but WRITE_DONE to user space are posted
  even before it receives FLUSH ack from DSP.
- Fix is to wait for FLUSH ack from DSP, then post the
  WRITE_DONE events to user space, which will ensure FLUSH
  is completely handled.

CRs-Fixed: 425420,458910
Change-Id: Ieca3353dba3a67af39a2967659034f073acf5277
Signed-off-by: Deepa Madiregama <dmadireg@codeaurora.org>