fpga manager: xilinx-spi: provide better diagnostics on programming failure

When the DONE pin does not go high after programming to confirm programming
success, the INIT_B pin provides some info on the reason. Use it if
available to provide a more explanatory error message.

Reviewed-by: Tom Rix <trix@redhat.com>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Moritz Fischer <mdf@kernel.org>