ALSA: scarlett2: Allow for controls with a "mute mode"

Gen 2/3 interfaces would only use 0/1 values for input level and
phantom power switch controls. Gen 4 interfaces use the second bit to
indicate that the state should be changed (or is changing), and the
input is to be muted (or is muted) while that happens.

Add a "mute" flag to config items to enable this behaviour for the
level/phantom controls.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/f603cd16079c97fad910087e0302828a289d1c15.1703612638.git.g@b4.vu