ALSA: scarlett2: Simplify linked channel handling

The current handling of linked channels for the 4th Gen 2i2 and 4i4
models is more complicated than necessary. The firmware has a link
control for each channel, and the channel selection can be any channel
regardless of the channel link status. The driver can therefore be
simplified by having it reflect the controls presented by the
firmware, and complexities related to trying to reflect the channel
link status in the Input Select Capture Enum control are not
necessary.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/9bf8a2ab35f243820d066319b7d407b45a61281b.1727971672.git.g@b4.vu