staging: comedi: quatech_daqp_cs: use comedi_offset_munge() in ao (*insn_write)

Use the comedi_offset_munge() helper to do the offset binary to two's
complement conversion when writing a new analog output value.
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/quatech_daqp_cs.c

@@ -655,11 +655,9 @@ static int daqp_ao_insn_write(struct comedi_device *dev,
 
 		s->readback[chan] = val;
 
-		val &= 0x0fff;
-		val ^= 0x0800;		/* Flip the sign */
-		val |= (chan << 12);
-
-		outw(val, dev->iobase + DAQP_DA);
+		/* write the two's complement value to the channel */
+		outw((chan << 12) | comedi_offset_munge(s, val),
+		     dev->iobase + DAQP_DA);
 	}
 
 	return insn->n;