Revert "auhal: fix data race with VolumeSet"

This reverts commit dcb42685.

My bad ! This commit is useless, the code and the documentation were
perfectly clear:

"This callback may be called concurrently with time_get(), play(),
pause() or flush(). It will however be protected against concurrent
calls to start(), stop(), volume_set(), mute_set() or device_select()."