pcm: hw: setup explicit silencing for snd_pcm_drain by default

pcm: hw: setup explicit silencing for snd_pcm_drain by default

Some applications may not alignt transfers to the period size
and also the driver developers may not follow the consequeces of the
access beyond valid samples in the playback DMA buffer.

To avoid clicks, fill a little silence at the end of the playback
ring buffer when snd_pcm_drain() is called.

Related: https://lore.kernel.org/alsa-devel/20230420113324.877164-2-oswald.buddenhagen@gmx.de/
Related: https://lore.kernel.org/alsa-devel/20230405201219.2197789-2-oswald.buddenhagen@gmx.de/
Signed-off-by: Jaroslav Kysela <perex@perex.cz>