staging: comedi: rti802: tidy up rti802_ao_insn_write()

Use comedi_offset_munge() to handle munging the offset binary to two's
complement.

Tidy up the function a bit.

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