staging: comedi: daqboard2000: change daqboard2000_write_cpld() return value

`daqboard2000_write_cpld()` currently returns 1 on success, or 0 on
failure.  Change it to return 0 on success, or `-EIO` on failure.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>