staging: comedi: gsc_hpdi: validate the cmd->start_arg

The cmd->start_arg should be trivially validated in Step 3 of the
(*do_cmdtest) functions. Add the necessary code.
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/gsc_hpdi.c

@@ -410,6 +410,8 @@ static int gsc_hpdi_cmd_test(struct comedi_device *dev,
 
 	/* Step 3: check if arguments are trivially valid */
 
+	err |= cfc_check_trigger_arg_is(&cmd->start_arg, 0);
+
 	if (!cmd->chanlist_len || !cmd->chanlist) {
 		cmd->chanlist_len = 32;
 		err |= -EINVAL;