staging: comedi: cb_pcidas: clarify the ai cmd->start_arg validation

This driver supports two cmd->start_src values, TRIG_NOW and TRIG_EXT.

For aesthetics, replace the default case when validating the cmd->start_arg
with the proper case statement.
Signed-off-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: NIan Abbott <abbotti@mev.co.uk>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/cb_pcidas.c

@@ -872,9 +872,12 @@ static int cb_pcidas_ai_cmdtest(struct comedi_device *dev,
 	if (err)
 		return 2;
 
-	/* step 3: arguments are trivially compatible */
+	/* Step 3: check if arguments are trivially valid */
 
 	switch (cmd->start_src) {
+	case TRIG_NOW:
+		err |= cfc_check_trigger_arg_is(&cmd->start_arg, 0);
+		break;
 	case TRIG_EXT:
 		/* External trigger, only CR_EDGE and CR_INVERT flags allowed */
 		if ((cmd->start_arg
@@ -888,9 +891,6 @@ static int cb_pcidas_ai_cmdtest(struct comedi_device *dev,
 			err |= -EINVAL;
 		}
 		break;
-	default:
-		err |= cfc_check_trigger_arg_is(&cmd->start_arg, 0);
-		break;
 	}
 
 	if (cmd->scan_begin_src == TRIG_TIMER)