staging: comedi: usbdux: clarify bipolar ai data

Use the comedi_range_is_bipolar() helper instead of checking the
'range' index against a magic number.

Also, use the s->maxdata to calculate the value needed to munge the
value for bipolar data instead of the magic number.
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/usbdux.c

@@ -1002,8 +1002,10 @@ static int usbdux_ai_insn_read(struct comedi_device *dev,
 			goto ai_read_exit;
 
 		val = le16_to_cpu(devpriv->insn_buffer[1]);
-		if (range <= 1)
-			val ^= 0x800;
+
+		/* bipolar data is two's-complement */
+		if (comedi_range_is_bipolar(s, range))
+			val ^= ((s->maxdata + 1) >> 1);
 
 		data[i] = val;
 	}