staging: comedi: usbduxsigma: round down AO scan_begin_arg at step 4.

The return value of the `cmdtest` handler for a subdevice checks the
prospective new command in various steps and returns the step number at
which any problem was detected, or 0 if no problem was detected.  It is
allowed to modify the command in various ways at each step.  Corrections
for out-of-range values are generally made at step 3, and minor
adjustments such as rounding are generally made at step 4.

The `cmdtest` handler for the AO subdevice (`usbduxsigma_ao_cmdtest()`)
currently range checks the timings at step 3.  Since the running command
will round down the timings, add code to round them down at step 4.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: Bernd Porr <mail@berndporr.me.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/usbduxsigma.c

@@ -839,6 +839,7 @@ static int usbduxsigma_ao_cmdtest(struct comedi_device *dev,
 				  struct comedi_cmd *cmd)
 {
 	struct usbduxsigma_private *devpriv = dev->private;
+	unsigned int tmp;
 	int err = 0;
 
 	/* Step 1 : check if triggers are trivially valid */
@@ -890,6 +891,14 @@ static int usbduxsigma_ao_cmdtest(struct comedi_device *dev,
 	if (err)
 		return 3;
 
+	/* Step 4: fix up any arguments */
+
+	tmp = rounddown(cmd->scan_begin_arg, 1000000);
+	err |= comedi_check_trigger_arg_is(&cmd->scan_begin_arg, tmp);
+
+	if (err)
+		return 4;
+
 	return 0;
 }