pcm: null plugin - fix the avail count
pcm: null plugin - fix the avail count

The automatic threshold start is not activated when the wrong avail
count is returned in the prepared state.

Signed-off-by: Jaroslav Kysela <72edd5e1a572f82f059ccbb83c92b74d6cb051ff@perex.cz>

takaswie/alsa-lib,zedongchen/alsa-lib,takaswie/alsa-lib,takaswie/alsa-lib,lgirdwood/alsa-lib,lgirdwood/alsa-lib,lgirdwood/alsa-lib,Distrotech/alsa-lib,mengdonglin/alsa-lib,tiwai/alsa-lib,mengdonglin/alsa-lib,takaswie/alsa-lib,lgirdwood/alsa-lib,tiwai/alsa-lib,Distrotech/alsa-lib,alsa-project/alsa-lib,lgirdwood/alsa-lib,zedongchen/alsa-lib,takaswie/alsa-lib,alsa-project/alsa-lib,zedongchen/alsa-lib,tiwai/alsa-lib,alsa-project/alsa-lib,alsa-project/alsa-lib,Distrotech/alsa-lib,zedongchen/alsa-lib,Distrotech/alsa-lib,mengdonglin/alsa-lib,zedongchen/alsa-lib,mengdonglin/alsa-lib,tiwai/alsa-lib,Distrotech/alsa-lib,mengdonglin/alsa-lib