ALSA: scarlett2: Rename db_scale_scarlett2_gain to volume

db_scale_scarlett2_gain is the TLV for the output volume controls.
Gen 4 has software-controllable input gain controls, so rename this to
db_scale_scarlett2_volume so we can use that name for the inputs.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/d544ec7cc5d5a849da104a5a78b17f61f50657c1.1703444932.git.g@b4.vu


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