ALSA: hda - Add mute-LED mode control to Thinkpad

This patch adds the missing flag to enable "Mute-LED Mode" mixer enum
ctl for Thinkpads that have also the software mute-LED control.

Reported-and-tested-by: Pali Rohár <pali.rohar@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>