The channel mapping is defined by bChRelationship, not bChPurpose.

Fixes: 9a2fe9b8 ("ALSA: usb: initial USB Audio Device Class 3.0 support")
Reviewed-by: NRuslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: NMichael Drake <michael.drake@codethink.co.uk>
Signed-off-by: NJorge Sanjuan <jorge.sanjuan@codethink.co.uk>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>