[netdevice] Allow devices to indicate that interrupts are not supported

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

src/include/ipxe/netdevice.h

 	 *
 	 * @v netdev	Network device
 	 * @v enable	Interrupts should be enabled
+	 *
+	 * This method may be NULL to indicate that interrupts are not
+	 * supported.
 	 */
 	void ( * irq ) ( struct net_device *netdev, int enable );
 };
 	return ( netdev->state & NETDEV_OPEN );
 }
 
+/**
+ * Check whether or not network device supports interrupts
+ *
+ * @v netdev		Network device
+ * @ret irq_supported	Network device supports interrupts
+ */
+static inline __attribute__ (( always_inline )) int
+netdev_irq_supported ( struct net_device *netdev ) {
+	return ( netdev->op->irq != NULL );
+}
+
 /**
  * Check whether or not network device interrupts are currently enabled
  *

src/net/netdevice.c

  */
 void netdev_irq ( struct net_device *netdev, int enable ) {
 
+	/* Do nothing if device does not support interrupts */
+	if ( ! netdev_irq_supported ( netdev ) )
+		return;
+
 	/* Enable or disable device interrupts */
 	netdev->op->irq ( netdev, enable );