gprs_ms: Use osmo_timer_setup() API

Change-Id: I261f4dcee15942bdce82c6b8a8a558a38bedd6fe
diff --git a/src/gprs_ms.c b/src/gprs_ms.c
index 6dc11a6..6daab31 100644
--- a/src/gprs_ms.c
+++ b/src/gprs_ms.c
@@ -111,8 +111,7 @@
 	LOGP(DRLCMAC, LOGL_INFO, "Creating MS object, TLLI = 0x%08x\n", tlli);
 
 	ms->imsi[0] = '\0';
-	memset(&ms->timer, 0, sizeof(ms->timer));
-	ms->timer.cb = ms_release_timer_cb;
+	osmo_timer_setup(&ms->timer, ms_release_timer_cb, NULL);
 	llc_queue_init(&ms->llc_queue, ms);
 
 	ms_set_mode(ms, GPRS);