ALSA: oxygen: Xonar DG(X): modify DAC routing

commit 1f91ecc14deea9461aca93273d78871ec4d98fcd upstream.

When selecting the audio output destinations (headphones, FP headphones,
multichannel output), unnecessary I2S channels are digitally muted to
avoid invalid signal levels on the other outputs.

Signed-off-by: Roman Volkov <v1ron@mail.ru>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>