usb: device_next: uac2: Generate Feature Unit descriptor

Rework NUM_SPATIAL_LOCATIONS() to evaluate to integer literal to allow
using it with LISTIFY() macro. This is necessary because Feature Unit do
not operate on channel clusters but rather on logical channels.

Track back Output Terminal channel cluster to appropriate entity. This
is necessary because Feature Unit does not repeat the channel cluster
information.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>