[pxe] Work around missing PXENV_UNDI_OPEN only when necessary

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

src/arch/i386/interface/pxe/pxe_undi.c

 	 * processing at this point, to work around callers that never
 	 * call PXENV_UNDI_OPEN before attempting to use the UNDI API.
 	 */
-	netdev_rx_freeze ( pxe_netdev );
-	netdev_irq ( pxe_netdev, 1 );
+	if ( ! netdev_rx_frozen ( pxe_netdev ) ) {
+		netdev_rx_freeze ( pxe_netdev );
+		netdev_irq ( pxe_netdev, 1 );
+	}
 
 	/* Identify network-layer protocol */
 	switch ( undi_transmit->Protocol ) {