[natsemi] Add missing iounmap()

Reported-by: Thomas Miletich <thomas.miletich@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/natsemi.c

  err_hwaddr:
 	natsemi_reset ( natsemi );
  err_reset:
+	iounmap ( natsemi->regs );
 	netdev_nullify ( netdev );
 	netdev_put ( netdev );
  err_alloc:
 	natsemi_reset ( natsemi );
 
 	/* Free network device */
+	iounmap ( natsemi->regs );
 	netdev_nullify ( netdev );
 	netdev_put ( netdev );
 }