ALSA: usb-audio: Add iD14 MkI to monitor-mix volume quirk

Add the iD14 MkI USB ID to the existing audient_id14_map. This
names feature unit 12 Monitor Mix Playback Volume.

Live meter A/B testing on MkI USB ID 2708:0002 reproduced the same
asymmetric attenuation seen on MkII. At ALSA value 104, the right
channel was 23.07 dB below the left. At 127, both channels were equal.

The misleading Speaker Playback Volume name causes PipeWire to adopt
FU12 as the stream master. Reuse the existing map to keep the control
available while preventing that selection.

Hardware-tested on Bazzite 7.2.0-ogc4.1.fc44.x86_64 with this change
backported. ALSA exposed Monitor Mix Playback Volume after module load
and USB reconnect. PipeWire volume changes from 100% to 35% and back
left all four FU12 values at 127. Channel-isolated left/right playback
remained balanced after reconnect.

Build-tested with CONFIG_SND_USB_AUDIO=m using the focused sound/usb
target.

Assisted-by: Codex:gpt-5
Signed-off-by: Will Porter <mrwillporter@gmail.com>
Link: https://patch.msgid.link/20260823015353.686080-1-mrwillporter@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

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