Fix bug reported by JP Carballo

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@3688 42af7a65-404d-4744-a932-0658087f49c3

include/queue.h

@@ -51,7 +51,7 @@
 
 #define sq_next(p) ((p)->flink)
 #define dq_next(p) ((p)->flink)
-#define dq_prev(p) ((p)->prev)
+#define dq_prev(p) ((p)->blink)
 
 #define sq_empty(q) ((q)->head == NULL)
 #define dq_empty(q) ((q)->head == NULL)