ALSA: hda - Add LFE chmap to ASUS ET2700

As the previous commit 1f0bbf03 added the pin config for the bass
speaker, this patch adds the corresponding LFE-only channel map on
ASUS ET2700.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=65961


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