Use the new common helper for setting up and controlling the mic mute
LED over thinkpad_acpi.  This also provides a new mixer enum "Mic
Mute-LED Mode" (that was present only for Dell models), which allows
user to choose the mic mute LED behavior.  For example, if you want
the mic mute LED turned on only while mic is on, choose "Follow
Capture" there.
Tested-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>