ALSA: usb-audio: map UAC3 front wide channels in convert_chmap_v3()

ALSA: usb-audio: map UAC3 front wide channels in convert_chmap_v3()

convert_chmap_v3() translates UAC3 channel relationship values into
ALSA channel-map positions.

UAC3_CH_FRONT_WIDE_LEFT and UAC3_CH_FRONT_WIDE_RIGHT currently fall
back to SNDRV_CHMAP_UNKNOWN, although ALSA already provides matching
channel-map positions via SNDRV_CHMAP_FLW and SNDRV_CHMAP_FRW.

Map these two UAC3 positions to their ALSA equivalents and update the
comment to clarify that unsupported UAC3 channel relationships remain
reported as SNDRV_CHMAP_UNKNOWN.

No functional change for other channel relationships.

Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Link: https://patch.msgid.link/20260311200010.103710-1-cassiogabrielcontato@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>