atusb/atusb.brd: some more silk screen cleanup
atusb/atusb.brd: some more silk screen cleanup

- atusb.brd (C22): properly aligned reference with component center
- atusb.brd (P11, P12, P13, P14): moved reference below the pad, for
  consistency with P15 and P16
- atusb.brd (CON1): centered the component reference

cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan