Fixed bug: mutex called from ISR.

Author: Frank Duerr

drivers/ethernet/eth_nxp_enet.c

@@ -164,15 +164,15 @@ static inline void ts_register_tx_event(const struct device *dev,
 
 	if (pkt && atomic_get(&pkt->atomic_ref) > 0) {
 		if (eth_get_ptp_data(net_pkt_iface(pkt), pkt) && frameinfo->isTsAvail) {
-			k_mutex_lock(data->ptp.ptp_mutex, K_FOREVER);
-
+		        // Timestamp is written in ISR.
+		        // Semaphore ptp_ts_sem ensures sequential execution of writing the timestamp
+		        // here and subsequently reading the timestamp after waiting for
+		        // ptp_ts_sem in eth_wait_for_ptp_ts().
 			pkt->timestamp.nanosecond = frameinfo->timeStamp.nanosecond;
 			pkt->timestamp.second = frameinfo->timeStamp.second;
 
 			net_if_add_tx_timestamp(pkt);
 			k_sem_give(&data->ptp.ptp_ts_sem);
-
-			k_mutex_unlock(data->ptp.ptp_mutex);
 		}
 		net_pkt_unref(pkt);
 	}