staging: comedi: ni_labpc: clarify the cmd->start_arg validation

This driver supports cmd->start_src sources of TRIG_NOW and TRIG_EXT.
Refactor the (*do_cmdtest) to clarify the trivial validation of the
cmd->start_arg.

This also fixes a bug, the cmd->start_src has the trigger not the
cmd->start_arg.
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/ni_labpc.c

@@ -649,8 +649,14 @@ static int labpc_ai_cmdtest(struct comedi_device *dev,
 
 	/* Step 3: check if arguments are trivially valid */
 
-	if (cmd->start_arg == TRIG_NOW)
+	switch (cmd->start_src) {
+	case TRIG_NOW:
 		err |= cfc_check_trigger_arg_is(&cmd->start_arg, 0);
+		break;
+	case TRIG_EXT:
+		/* start_arg value is ignored */
+		break;
+	}
 
 	if (!cmd->chanlist_len)
 		err |= -EINVAL;