ALSA: scarlett2: Add Vocaster speaker/headphone mute controls

Add support for the speaker and headphone mute controls on Focusrite
Vocaster interfaces. Unlike other Focusrite interfaces, these mute
controls are per-output, not per-channel.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://patch.msgid.link/aLm2Dy6+O/eLTGVn@m.b4.vu


Signed-off-by: Takashi Iwai <tiwai@suse.de>