net: ptp: calculate link delay with right timestamps

At the starting of ptp syncrhonization, there may be not Sync frame
TX/RX timestamps in first time link delay calculation.
So, need a check for that in case of wrong link delay calculated.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>

Author: yangbolu1991

subsys/net/lib/ptp/clock.c

@@ -556,6 +556,10 @@ void ptp_clock_delay(uint64_t egress, uint64_t ingress)
 {
 	int64_t delay;
 
+	if (ptp_clk.timestamp.t1 == 0 || ptp_clk.timestamp.t2 == 0) {
+		return;
+	}
+
 	ptp_clk.timestamp.t3 = egress;
 	ptp_clk.timestamp.t4 = ingress;