staging: comedi: das16: use comedi_subdevice 'readback'

Use the new comedi_subdevice 'readback' member and the core provided
(*insn_read) for the readback of the analog output subdevice channels.
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/das16.c

@@ -927,11 +927,13 @@ static int das16_ao_insn_write(struct comedi_device *dev,
 			       unsigned int *data)
 {
 	unsigned int chan = CR_CHAN(insn->chanspec);
-	unsigned int val;
 	int i;
 
 	for (i = 0; i < insn->n; i++) {
-		val = data[i];
+		unsigned int val = data[i];
+
+		s->readback[chan] = val;
+
 		val <<= 4;
 
 		outb(val & 0xff, dev->iobase + DAS16_AO_LSB_REG(chan));
@@ -1163,6 +1165,11 @@ static int das16_attach(struct comedi_device *dev, struct comedi_devconfig *it)
 		s->maxdata	= 0x0fff;
 		s->range_table	= devpriv->user_ao_range_table;
 		s->insn_write	= das16_ao_insn_write;
+		s->insn_read	= comedi_readback_insn_read;
+
+		ret = comedi_alloc_subdev_readback(s);
+		if (ret)
+			return ret;
 	} else {
 		s->type		= COMEDI_SUBD_UNUSED;
 	}