[netdevice] Add find_netdev_by_index()

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/netdevice.h

 extern void unregister_netdev ( struct net_device *netdev );
 extern void netdev_irq ( struct net_device *netdev, int enable );
 extern struct net_device * find_netdev ( const char *name );
+extern struct net_device * find_netdev_by_index ( unsigned int index );
 extern struct net_device * find_netdev_by_location ( unsigned int bus_type,
 						     unsigned int location );
 extern struct net_device * last_opened_netdev ( void );

src/net/netdevice.c

 	return NULL;
 }
 
+/**
+ * Get network device by index
+ *
+ * @v index		Network device index
+ * @ret netdev		Network device, or NULL
+ */
+struct net_device * find_netdev_by_index ( unsigned int index ) {
+	struct net_device *netdev;
+
+	/* Identify network device by index */
+	list_for_each_entry ( netdev, &net_devices, list ) {
+		if ( netdev->index == index )
+			return netdev;
+	}
+
+	return NULL;
+}
+
 /**
  * Get network device by PCI bus:dev.fn address
  *