Previously, we had two separate controls; there's no need to have AD1888's
HPF control, so drop it if we're on an OLPC machine.  Also, as per Arjun's
request, rename OLPC's Analog Input Switch control to "DC Mode Enable".
Signed-off-by: Andres Salomon <dilinger@debian.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>