usb: rework to use macro STRUCT_SECTION_FOREACH

Replace deprecated macro USBD_CFG_DATA_DEFINE by
USBD_DEFINE_CFG_DATA which places usb_cfg_data structures
in specific iterable section.

Replace __usb_data_start, __usb_data_end usage patterns
  size_t size = (__usb_data_end - __usb_data_start);
  for (size_t i = 0; i < size; i++) {...}
by
  STRUCT_SECTION_FOREACH(usb_cfg_data, ...) {...}

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>