[http] Notify data transfer interface when underlying connection is ready

HTTP implements xfer_window_changed() on the underlying server
connection using http_step(), which does not propagate the window
change notification to the data transfer interface.  This breaks the
multipath-capable SAN boot code, which relies on the window change
notification to discover that the HTTP block device is ready for
commands to be issued.

Fix by sending xfer_window_changed() in http_step() once the
underlying connection has been determined to be ready.

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

src/net/tcp/httpcore.c

 	if ( ! xfer_window ( &http->conn ) )
 		return;
 
+	/* Notify data transfer interface that window may have changed */
+	xfer_window_changed ( &http->xfer );
+
 	/* Do nothing until data transfer interface is ready */
 	if ( ! xfer_window ( &http->xfer ) )
 		return;