Add XFER_INIT() macro.

src/core/xfer.c

  * connected when unplugged.  It will never generate messages, and
  * will silently absorb all received messages.
  */
-struct xfer_interface null_xfer = {
-	.intf = {
-		.dest = &null_xfer.intf,
-		.refcnt = NULL,
-	},
-	.op = &null_xfer_ops,
-};
+struct xfer_interface null_xfer = XFER_INIT ( &null_xfer_ops );

src/include/gpxe/xfer.h

 	xfer->op = op;
 }
 
+/**
+ * Initialise a static data transfer interface
+ *
+ * @v operations		Data transfer interface operations
+ */
+#define XFER_INIT( operations ) {			\
+		.intf = {				\
+			.dest = &null_xfer.intf,	\
+			.refcnt = NULL,			\
+		},					\
+		.op = operations,			\
+	}
+
 /**
  * Get data transfer interface from generic object communication interface
  *