tbf: Move the timer routine into the class

The timer is used for various timeouts and there is still external
client code that is calling it. In a perfect world the client code
would indicate that an event has happened and the internal timer
will be stopped. The best compromise is the "stop_t3191" method. It
allows to add semantic verification that the timer has been running.
diff --git a/src/gprs_rlcmac_data.cpp b/src/gprs_rlcmac_data.cpp
index 035c203..d5bebff 100644
--- a/src/gprs_rlcmac_data.cpp
+++ b/src/gprs_rlcmac_data.cpp
@@ -928,7 +928,7 @@
 		tbf_new_state(new_tbf, GPRS_RLCMAC_FLOW);
 		tbf_assign_control_ts(new_tbf);
 		/* stop pending assignment timer */
-		tbf_timer_stop(new_tbf);
+		new_tbf->stop_timer();
 
 	}
 	debug_diagram(bts->bts, tbf->diag, "send DL-ASS");
@@ -968,7 +968,7 @@
 #endif
 
 	/* stop pending timer */
-	tbf_timer_stop(tbf);
+	tbf->stop_timer();
 
 	/* check for downlink tbf:  */
 	if (old_tbf) {