ALSA: pcm: use exit controlled loop in snd_pcm_playback_silence()

We already know that `frames` is greater than zero, because we just
checked it. So we don't need to check the loop condition on the first
iteration.

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Link: https://lore.kernel.org/r/20230505155244.2312199-7-oswald.buddenhagen@gmx.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>