tbf: update(): return negative val on error

Let's follow usual convention where errors are returned as negative
values.

Change-Id: Ib4f4dc37ae82ba8efdc212ed85af7934e16a8a8d
diff --git a/src/tbf.cpp b/src/tbf.cpp
index 3e190b4..50940c8 100644
--- a/src/tbf.cpp
+++ b/src/tbf.cpp
@@ -323,7 +323,7 @@
 	if (rc < 0) {
 		LOGPTBF(this, LOGL_ERROR, "No resource after update???\n");
 		bts_do_rate_ctr_inc(bts, CTR_TBF_ALLOC_FAIL);
-		return -rc;
+		return rc;
 	}
 
 	if (is_egprs_enabled()) {