userspace/include/fpga_io.h: reindent to make it more readable

This header is almost obsolete, as we'll move to SDB soon.
Meanwhile, sort out the tab/space mess and indent with spaces only,
so it looks good to everybody (including greg who insists on one
tab every 2 spaces -- which works for indentation, but not for
for alignment at end-of-line).
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>