cfq-iosched: Adjust one function call together with a variable assignment

The script "checkpatch.pl" pointed information out like the following.

ERROR: do not use assignment in if condition

Thus fix the affected source code place.
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/cfq-iosched.c

@@ -2749,9 +2749,11 @@ static struct cfq_queue *cfq_get_next_queue_forced(struct cfq_data *cfqd)
 	if (!cfqg)
 		return NULL;
 
-	for_each_cfqg_st(cfqg, i, j, st)
-		if ((cfqq = cfq_rb_first(st)) != NULL)
+	for_each_cfqg_st(cfqg, i, j, st) {
+		cfqq = cfq_rb_first(st);
+		if (cfqq)
 			return cfqq;
+	}
 	return NULL;
 }