ALSA: hda/realtek: Limit Legion AW88399 playback to stereo

The Legion AW88399 speaker routing sends a stereo FL/FR stream
to both speaker pairs. A four-channel stream leaves the front
pair silent, so advertising four channels exposes an unusable
playback mode.

Limit the analogue PCM and the multi-output runtime constraint
to two channels for the affected Legion codec SSIDs. This
exposes the usable stereo configuration and rejects
four-channel playback.

Signed-off-by: Aaron Ma <mapengyu@gmail.com>
Reviewed-by: Marco Giunta <marco_giunta@outlook.it>
Tested-by: Marco Giunta <marco_giunta@outlook.it>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20260804185325.22861-2-mapengyu@gmail.com