TBF: log source of state transitions

We use the same approach for osmo_fsm: when state transition happens,
it's not very useful to always log the transition function itself, it's
much more useful to see where the actual transition comes from.

Change-Id: I348ba89bdda2b44c7019e9c893c764ee08c80bec
Related: OS#1759
diff --git a/src/tbf_ul.cpp b/src/tbf_ul.cpp
index 83ac08f..eaf9c93 100644
--- a/src/tbf_ul.cpp
+++ b/src/tbf_ul.cpp
@@ -330,7 +330,7 @@
 		if (rdbi->cv == 0) {
 			LOGP(DRLCMACUL, LOGL_DEBUG, "- Finished with UL "
 				"TBF\n");
-			set_state(GPRS_RLCMAC_FINISHED);
+			TBF_SET_STATE(this, GPRS_RLCMAC_FINISHED);
 			/* Reset N3103 counter. */
 			this->m_n3103 = 0;
 		}