ALSA: scarlett2: Add support for air/phantom control on input 2

The Focusrite Scarlett Gen 4 Solo has Air and Phantom Power controls
on analogue input thesofproject#2 (the Gen 3 Solo had these controls on analogue
input #1). Add air_input_first and phantom_first device info options
to cater for this.

These options are similar to the level_input_first option that was
added for the Gen 3 Solo, but these new options do not require
adjusting the index of the control.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/19511f18895b8c094985a4a5691fbc1dc028c108.1703612638.git.g@b4.vu

sound/usb/mixer_scarlett2.c

@@ -632,9 +632,15 @@ struct scarlett2_device_info {
  */
  u8 air_input_count;
 
+ /* the first input with an air control (0-based) */
+ u8 air_input_first;
+
  /* the number of phantom (48V) software switchable controls */
  u8 phantom_count;
 
+ /* the first input with phantom power control (0-based) */
+ u8 phantom_first;
+
  /* the number of inputs each phantom switch controls */
  u8 inputs_per_phantom;
 
@@ -3043,6 +3049,7 @@ static int scarlett2_phantom_ctl_put(struct snd_kcontrol *kctl,
  struct usb_mixer_elem_info *elem = kctl->private_data;
  struct usb_mixer_interface *mixer = elem->head.mixer;
  struct scarlett2_data *private = mixer->private_data;
+ const struct scarlett2_device_info *info = private->info;
 
  int index = elem->control;
  int oval, val, err = 0;
@@ -3064,7 +3071,7 @@ static int scarlett2_phantom_ctl_put(struct snd_kcontrol *kctl,
 
  /* Send switch change to the device */
  err = scarlett2_usb_set_config(mixer, SCARLETT2_CONFIG_PHANTOM_SWITCH,
- index, val);
+ index + info->phantom_first, val);
  if (err == 0)
  err = 1;
 
@@ -3763,7 +3770,8 @@ static int scarlett2_add_line_in_ctls(struct usb_mixer_interface *mixer)
 
  /* Add input air controls */
  for (i = 0; i < info->air_input_count; i++) {
- snprintf(s, sizeof(s), fmt, i + 1, "Air", "Switch");
+ snprintf(s, sizeof(s), fmt, i + 1 + info->air_input_first,
+ "Air", "Switch");
  err = scarlett2_add_new_ctl(mixer, &scarlett2_air_ctl,
  i, 1, s, &private->air_ctls[i]);
  if (err < 0)
@@ -3773,7 +3781,8 @@ static int scarlett2_add_line_in_ctls(struct usb_mixer_interface *mixer)
  /* Add input phantom controls */
  if (info->inputs_per_phantom == 1) {
  for (i = 0; i < info->phantom_count; i++) {
- scnprintf(s, sizeof(s), fmt, i + 1,
+ scnprintf(s, sizeof(s), fmt,
+ i + 1 + info->phantom_first,
  "Phantom Power", "Switch");
  err = scarlett2_add_new_ctl(
  mixer, &scarlett2_phantom_ctl,