usb: device_next: uac2: Do not leak double buffered endpoint

UDC drivers use udc_buf_get_all() when dequeueing endpoint. On drivers
that require double buffering for isochronous IN endpoint, the dequeue
on interface disable will result in class request API called on net_buf
with frags set. Call release callback on the buffer pointed in frags
because it was passed in using usbd_uac2_send().

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