ALSA: scarlett2: Rename DSP mux channels

The DSP mux channels are of type SCARLETT2_PORT_TYPE_MIX so the
ALSA controls would refer to them "Mix X" and "Mixer Input X". This
patch fixes them to be called "DSP X" and "DSP Input X".

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/2d91d0a74d5c7f6179e950bed2c80a4498d16649.1703612638.git.g@b4.vu