staging: comedi: das1800: use comedi_offset_munge() for analog output

The analog outputs expect 2's complement data. For aesthetics, use
the helper function to handle the munging instead of depending on
the boardinfo 'resolution'.
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>

drivers/staging/comedi/drivers/das1800.c

@@ -1132,8 +1132,7 @@ static int das1800_ao_insn_write(struct comedi_device *dev,
 	unsigned short output;
 	unsigned long irq_flags;
 
-	/*   card expects two's complement data */
-	output = data[0] - (1 << (board->resolution - 1));
+	output = comedi_offset_munge(s, data[0]);
 	/*  if the write is to the 'update' channel, we need to remember its value */
 	if (chan == update_chan)
 		devpriv->ao_update_bits = output;