Use proper API osmo_timer_setup() to set up timer struct

Change-Id: Idd2b0c5247870bee3b3c3e460a6731ee50a47404
diff --git a/src/gprs_bssgp_pcu.cpp b/src/gprs_bssgp_pcu.cpp
index ddf5174..ce56154 100644
--- a/src/gprs_bssgp_pcu.cpp
+++ b/src/gprs_bssgp_pcu.cpp
@@ -946,8 +946,7 @@
 
 	osmo_signal_register_handler(SS_L_NS, nsvc_signal_cb, NULL);
 
-	the_pcu.bvc_timer.cb = bvc_timeout;
-
+	osmo_timer_setup(&the_pcu.bvc_timer, bvc_timeout, NULL);
 
 	return &the_pcu;
 }