[ALSA] AD1888 mixer controls for DC mode

This patch adds two mixer controls. The V_REFOUT enable is a
documented register that couples the microphone input lines
to the V_REFOUT DC source. The High Pass Filter enable in the
AC97_AD_TEST2 (0x5c) is an undocumented register provided by
Miller Puckette via Analog Devices that enables the AD codec
to apply a high pass filter to the input.

Signed-off-by: Jaya Kumar <jayakumar.alsa@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>