[myson] Add missing iounmap()

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

src/drivers/net/myson.c

  err_register_netdev:
 	myson_reset ( myson );
  err_reset:
+	iounmap ( myson->regs );
 	netdev_nullify ( netdev );
 	netdev_put ( netdev );
  err_alloc:
 	myson_reset ( myson );
 
 	/* Free network device */
+	iounmap ( myson->regs );
 	netdev_nullify ( netdev );
 	netdev_put ( netdev );
 }