eth/filters: make filterLogs func more readable (ethereum#16920)

Author: nujeh

eth/filters/filter.go

@@ -258,9 +258,9 @@ Logs:
 		if len(topics) > len(log.Topics) {
 			continue Logs
 		}
-		for i, topics := range topics {
-			match := len(topics) == 0 // empty rule set == wildcard
-			for _, topic := range topics {
+		for i, sub := range topics {
+			match := len(sub) == 0 // empty rule set == wildcard
+			for _, topic := range sub {
 				if log.Topics[i] == topic {
 					match = true
 					break