register_serial -- configure a 16x50 serial port at runtime
(struct serial_struct * req);
Configure the serial port specified by the request. If the
port exists and is in use an error is returned. If the port
is not currently in the table it is added.
The port is then probed and if neccessary the IRQ is autodetected
If this fails an error is returned.
On success the port is ready to use and the line number is returned.