ALSA: usb-audio: ADC3: Fix channel mapping conversion for ADC3.

The channel mapping is defined by bChRelationship, not bChPurpose.

Fixes: 9a2fe9b8

 ("ALSA: usb: initial USB Audio Device Class 3.0 support")
Reviewed-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: Michael Drake <michael.drake@codethink.co.uk>
Signed-off-by: Jorge Sanjuan <jorge.sanjuan@codethink.co.uk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>