staging: r8188eu: rewrite subtraction in core/rtw_cmd.c

Rewrite subtraction in core/rtw_cmd.c to clear missing spaces around
operator reported by checkpatch.

256-1 -> 255

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20210816155818.24005-3-straube.linux@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>