Since the recent code change, the conditional build with CONFIG_PM is
calling only snd_power_sync_ref().  As a dummy function is provided
for this function, we can get rid of CONFIG_PM gracefully now.

Link: https://lore.kernel.org/r/20240510125128.6058-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>