ALSA: control-led: Replace sprintf() with sysfs_emit()

For sysfs outputs, it's safer to use a new helper, sysfs_emit(),
instead of the raw sprintf() & co.  This patch replaces such sprintf()
calls with sysfs_emit() while simplifying the open code in
list_show().

Link: https://lore.kernel.org/r/20220801165639.26030-4-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>