fix bug90145

Change-Id: Id59316a7f2cb68aba75e24a19a226d6639b6e5e7
diff --git a/mbtk/libmbtk_lib/common/mbtk_at.c b/mbtk/libmbtk_lib/common/mbtk_at.c
index 8262756..357e1f6 100755
--- a/mbtk/libmbtk_lib/common/mbtk_at.c
+++ b/mbtk/libmbtk_lib/common/mbtk_at.c
@@ -156,7 +156,7 @@
 {
     if(at_fd < 0) {
         LOGW("MBTK AT not inited.");
-        return 0;
+        return -1;
     }
 
     close(at_fd);