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 #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