Dell laptops have another LED for mic-mute in addition to the master
mute.  The former is tied with the capture switch (in a reverse way)
while the latter is tied with the master playback switch.  We already
have an enum control to change the behavior for the master mute LED in
different ways, e.g. keeping always off or turning off at mute.  But,
the mic-mute LED has no such management but its behavior is
hard-coded.

This patch implements an enum control to change the mic-mute LED
behavior like what we have for the master mute LED.  The ctl provides
four modes: keep-on, keep-off, follow-capture and follow-mute.  The
default mode is the last one, follow-mute, which follows the capture
mute, i.e. LED turning on when the capture is off, and turning off
when the capture is active.
Signed-off-by: Takashi Iwai <tiwai@suse.de>