ALSA: core: Replace scnprintf() with sysfs_emit()

sysfs_emit() is a new helper to simplify the sysfs string output.
Replace the open-code with this new helper.

Link: https://lore.kernel.org/r/20220801165639.26030-5-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>