net: ptp: clock: Create a dummy inline func for net_eth_get_ptp_clock()

Follow the style in other similar functions in this file.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>

include/net/ethernet.h

@@ -671,7 +671,16 @@ int net_eth_promisc_mode(struct net_if *iface, bool enable);
  * @return Pointer to PTP clock if found, NULL if not found or if this
  * ethernet interface does not support PTP.
  */
+#if defined(CONFIG_PTP_CLOCK)
 struct device *net_eth_get_ptp_clock(struct net_if *iface);
+#else
+static inline struct device *net_eth_get_ptp_clock(struct net_if *iface)
+{
+	ARG_UNUSED(iface);
+
+	return NULL;
+}
+#endif
 
 /**
  * @brief Return gPTP port number attached to this interface.

subsys/net/l2/ethernet/ethernet.c

@@ -965,9 +965,9 @@ void net_eth_carrier_off(struct net_if *iface)
 	handle_carrier(ctx, iface, carrier_off);
 }
 
+#if defined(CONFIG_PTP_CLOCK)
 struct device *net_eth_get_ptp_clock(struct net_if *iface)
 {
-#if defined(CONFIG_PTP_CLOCK)
 	struct device *dev = net_if_get_device(iface);
 	const struct ethernet_api *api = dev->driver_api;
 
@@ -984,10 +984,8 @@ struct device *net_eth_get_ptp_clock(struct net_if *iface)
 	}
 
 	return api->get_ptp_clock(net_if_get_device(iface));
-#else
-	return NULL;
-#endif
 }
+#endif /* CONFIG_PTP_CLOCK */
 
 #if defined(CONFIG_NET_GPTP)
 int net_eth_get_ptp_port(struct net_if *iface)