staging: comedi: rti800: use comedi_offset_munge()

Use the comedi_offset_munge() helper to do the two's complement
to comedi offset binary and comedi offset binary to two's complement
conversions.
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/rti800.c

@@ -189,17 +189,21 @@ static int rti800_ai_insn_read(struct comedi_device *dev,
 	}
 
 	for (i = 0; i < insn->n; i++) {
+		unsigned int val;
+
 		outb(0, dev->iobase + RTI800_CONVERT);
 
 		ret = comedi_timeout(dev, s, insn, rti800_ai_eoc, 0);
 		if (ret)
 			return ret;
 
-		data[i] = inb(dev->iobase + RTI800_ADCLO);
-		data[i] |= (inb(dev->iobase + RTI800_ADCHI) & 0xf) << 8;
+		val = inb(dev->iobase + RTI800_ADCLO);
+		val |= (inb(dev->iobase + RTI800_ADCHI) & 0xf) << 8;
 
 		if (devpriv->adc_2comp)
-			data[i] ^= 0x800;
+			val = comedi_offset_munge(s, val);
+
+		data[i] = val;
 	}
 
 	return insn->n;
@@ -222,7 +226,7 @@ static int rti800_ao_insn_write(struct comedi_device *dev,
 		s->readback[chan] = val;
 
 		if (devpriv->dac_2comp[chan])
-			val ^= 0x800;
+			val = comedi_offset_munge(s, val);
 
 		outb(val & 0xff, dev->iobase + reg_lo);
 		outb((val >> 8) & 0xff, dev->iobase + reg_hi);