ALSA: scarlett2: Add support for software-controllable input gain

Some devices in the Scarlett Gen 4 series have support for
software-controllable input gain. Along with this comes a channel
select option, an auto-gain feature, "safe" mode, and linking two
channels into a stereo pair.

Mark the new scarlett2_notify_input_*() functions with __always_unused
until they get used when the Gen 4 notification callback function
arrays are added.
Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/fc39e80bb39863dd1579d342097203942b4f3034.1703612638.git.g@b4.vu