hdspmixer: Introduce output channel count for all cards.

hdspmixer: Introduce output channel count for all cards.

Some cards (like multiface) have more physical output ports than
playback ports, mostly because of additional headphones out.

For those cards, the old abstraction model of channels_input and
channels_output doesn't fit, so let's introduce channels_output.

Of course, channels_output is always 2*max_dest at the given speed_mode
(SS/DS/QS), so one could extend this idea, store all destination
settings in channels_output[3] (one for each speed mode) and rip off the
massive code duplication for setting maxdest or max_dest respectively.

Note that dest_map_whatever_speed_mode's array size indirectly defines
the right value for channels_output (read: even more unwanted
redundancy)

Signed-off-by: Adrian Knoth <adi@drcomp.erfurt.thur.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>