fpga: Fix Spartan II FPGA booting

fpga: Fix Spartan II FPGA booting

This patch does some minor fixing of the Xilinx Spartan II
FPGA boot code:

- Fixed call order of post configuration callback and
  success message printing (result of copy-paste?)
- relocate post configuration callback only when it
  is implemented
- remove obsolete comment
- minor coding style cleanup

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd-electronics.com>