ALSA: scarlett2: Add S/PDIF source selection controls

ALSA: scarlett2: Add S/PDIF source selection controls

Add S/PDIF Source/Digital I/O Mode selection controls for the Scarlett
3rd Gen 18i8/18i20 and Clarett 4Pre/8Pre interfaces. These models have
both coax S/PDIF and optical inputs, and the optical inputs are
switchable between being used as S/PDIF and ADAT inputs. The Scarlett
3rd Gen 18i20 also has a "Dual ADAT" mode for 8-channel audio at
88.2/96kHz.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/Zj8zCTjzPsTDENN+@m.b4.vu
Signed-off-by: Takashi Iwai <tiwai@suse.de>