Img_utils: Fix "unused" variables

For build-system CFLAGS clean-up, fix unused variables.

Change-Id: I589a73780f022271fce40f64305b29a84c382e5f
diff --git a/media/img_utils/src/FileInput.cpp b/media/img_utils/src/FileInput.cpp
index 498e715..4c85a51 100644
--- a/media/img_utils/src/FileInput.cpp
+++ b/media/img_utils/src/FileInput.cpp
@@ -78,7 +78,7 @@
         ret = BAD_VALUE;
     }
     mOpen = false;
-    return OK;
+    return ret;
 }
 
 } /*namespace img_utils*/