Clarify behaviour of plug() by using intf_put() and intf_get().

src/core/interface.c

  * interface into a null interface.
  */
 void plug ( struct interface *intf, struct interface *dest ) {
-	ref_put ( intf->dest->refcnt );
-	intf->dest = dest;
-	ref_get ( intf->dest->refcnt );
+	intf_put ( intf->dest );
+	intf->dest = intf_get ( dest );
 }
 
 /**

src/include/gpxe/interface.h

 	struct refcnt *refcnt;
 };
 
+/**
+ * Increment reference count on an interface
+ *
+ * @v intf		Interface
+ * @ret intf		Interface
+ */
+static inline struct interface * intf_get ( struct interface *intf ) {
+	ref_get ( intf->refcnt );
+	return intf;
+}
+
+/**
+ * Decrement reference count on an interface
+ *
+ * @v intf		Interface
+ */
+static inline void intf_put ( struct interface *intf ) {
+	ref_put ( intf->refcnt );
+}
+
 extern void plug ( struct interface *intf, struct interface *dest );
 extern void plug_plug ( struct interface *a, struct interface *b );