ALSA: scarlett2: Add support for DSP mux channels

The DSP mux channels in the Scarlett 4th Gen appear as
SCARLETT2_PORT_TYPE_MIX ports but do not have corresponding mixer
controls. Add a dsp_count option to the device info struct to exclude
those DSP channels from the num_mix_in/num_mix_out counts.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/b78bdb1a7624d55783f5bf0e1ffbfa47a9e9a800.1703612638.git.g@b4.vu