ALSA: oxygen: Xonar DG(X): fix Stereo Upmixing regression

The code introduced in commit 1f91ecc14dee ("ALSA: oxygen: modify
adjust_dg_dac_routing function") accidentally disregarded the old value
of the playback routing register, so it broke the "Stereo Upmixing"
mixer control.

The unmuted parts of the channel routing are the same for all settings
of the output destination, so it suffices to revert that part of the
patch.

Fixes: 1f91ecc14dee ('ALSA: oxygen: modify adjust_dg_dac_routing function')
Tested-by: Roman Volkov <v1ron@mail.ru>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>