The consenus is that the best default address for Xosera
is $A0, so make that the default.
However, in that case we need to limit the number of UARTs
that are probed to two because the probe for a third UART
writes unlucky values to Xosera (at $A3) and causes it to
reconfigure itself and lock up the bus for a time.
Thanks to Wayne for his help in debugging this.
Rob Gowin