Use net_device_operations structure and netdev_nullify() to allow for

safe dropping of the netdev ref by the driver while other refs still
exist.

Add netdev_irq() method.  Net device open()/close() methods should no
longer enable or disable IRQs.

Remove rx_quota; it wasn't used anywhere and added too much complexity
to implementing correct interrupt-masking behaviour in pxe_undi.c.

src/net/nullnet.c

@@ -0,0 +1,57 @@
+/*
+ * Copyright (C) 2006 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#include <stdint.h>
+#include <gpxe/iobuf.h>
+#include <gpxe/netdevice.h>
+
+/** @file
+ *
+ * Null network device
+ *
+ */
+
+static int null_open ( struct net_device *netdev __unused ) {
+	return -ENODEV;
+};
+
+static void null_close ( struct net_device *netdev __unused ) {
+	/* Do nothing */
+};
+
+static int null_transmit ( struct net_device *netdev __unused,
+			   struct io_buffer *iobuf __unused ) {
+	return -ENODEV;
+};
+
+static void null_poll ( struct net_device *netdev __unused ) {
+	/* Do nothing */
+}
+
+static void null_irq ( struct net_device *netdev __unused,
+		       int enable __unused ) {
+	/* Do nothing */
+}
+
+struct net_device_operations null_netdev_operations = {
+	.open		= null_open,
+	.close		= null_close,
+	.transmit	= null_transmit,
+	.poll		= null_poll,
+	.irq   		= null_irq,
+};