staging: comedi: ni_mio_common: Use insn->n in ni_calib_insn_write()

The `insn_write` handler for the calibration subdevice
(`ni_calib_insn_write()`) currently ignores `insn->n` (the number of
samples to write) and assumes a single sample is to be written, but
`insn->n` could be 0, meaning no samples should be written, in which
case `data[0]` is invalid.

Change `ni_calib_insn_write()` to only write to the calibration device
if `insn->n > 0`.  There isn't much point writing all the values when
`insn->n > 1`, so just write the last one (`data[insn->n - 1]`).

Also follow the usual Comedi convention and return `insn->n` from the
handler to indicate success (although any non-negative return value will
do as far as the Comedi core is concerned).
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/ni_mio_common.c

@@ -4394,9 +4394,13 @@ static int ni_calib_insn_write(struct comedi_device *dev,
 			       struct comedi_insn *insn,
 			       unsigned int *data)
 {
-	ni_write_caldac(dev, CR_CHAN(insn->chanspec), data[0]);
+	if (insn->n) {
+		/* only bother writing the last sample to the channel */
+		ni_write_caldac(dev, CR_CHAN(insn->chanspec),
+				data[insn->n - 1]);
+	}
 
-	return 1;
+	return insn->n;
 }
 
 static int ni_calib_insn_read(struct comedi_device *dev,