int uart_register_port (struct uart_driver * drv, struct uart_port * port);
Register UART settings with the specified low level driver. Detect the type of the port if UPF_BOOT_AUTOCONF is set, and detect the IRQ if UPF_AUTO_IRQ is set.
We try to pick the same port for the same IO base address, so that when a modem is plugged in, unplugged and plugged back in, it gets allocated the same port.
Returns negative error, or positive line number.