ALSA: pcm: playback silence - move silence variable updates to separate function

The code tracking the added samples in thresholded mode and the code
tracking the just played samples in top-up mode are semantically
identical, so factor it out to a common function to enhance readability.

Co-developed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230505155244.2312199-5-oswald.buddenhagen@gmx.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>