staging: gs_fpgaboot: io.c: Remove unused function

Remove the function bitswap() that is not used anywhere.

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Reviewed-by: Insop Song <Insop.Song@gainspeed.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>