TSCH: fix a wrong return value

tsch_security_secure_frame() returns the length of MIC in case of
success. Otherwise, it is supposed to return 0. There was one case where
it returns 1 in an error handling block.

core/net/mac/tsch/tsch-security.c

@@ -160,7 +160,7 @@ tsch_security_secure_frame(uint8_t *hdr, uint8_t *outbuf,
 
   if(!frame.fcf.security_enabled) {
     /* Security is not enabled for this frame, we're done */
-    return 1;
+    return 0;
   }
 
   /* Read security key index */