Fix position buffer updates to use the correct stream offset.

Without this patch both IN (record) and OUT (playback) streams
will update the IN buffer positions.  The linux kernel notices
and complains:
  hda-intel: Invalid position buffer, using LPIB read method instead.

The bug may also lead to glitches when recording and playing
at the same time:
  https://bugzilla.redhat.com/show_bug.cgi?id=947785

Cc: qemu-stable@nongnu.org
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>