ALSA: line6: 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 those usages
straightforwardly with a new helper, sysfs_emit().

Link: https://lore.kernel.org/r/20220801165639.26030-8-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>