ALSA: virtuoso: Xonar DS: add stereo upmixing to center/LFE channels

Add the possibility to route a mix of the two channels of stereo data to
the center and LFE outputs.  Due to a WM8766 restriction, all surround
and back channels also get the mixed L/R signal in this case.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>