[gprs] NS: replace nsvc->timer_is_tns_alive with nsvc->timer_mode

This will allow to use the timer in more than 2 modes
diff --git a/openbsc/include/openbsc/gprs_ns.h b/openbsc/include/openbsc/gprs_ns.h
index 573536d..4078404 100644
--- a/openbsc/include/openbsc/gprs_ns.h
+++ b/openbsc/include/openbsc/gprs_ns.h
@@ -118,6 +118,14 @@
 	};
 };
 
+enum nsvc_timer_mode {
+	/* standard timers */
+	NSVC_TIMER_TNS_TEST,
+	NSVC_TIMER_TNS_ALIVE,
+	/* custom timer */
+	NSVC_TIMER_RESET,
+};
+
 struct gprs_nsvc {
 	struct llist_head list;
 	struct gprs_ns_inst *nsi;
@@ -128,8 +136,8 @@
 	uint32_t state;
 	uint32_t remote_state;
 
-	struct timer_list alive_timer;
-	int timer_is_tns_alive;
+	struct timer_list timer;
+	enum nsvc_timer_mode timer_mode;
 	int alive_retries;
 
 	int remote_end_is_sgsn;