[SERIAL] Prevent PNPBIOS re-registering already detected ports

During initialisation of 8250 serial, we scan a list of ISA ports and
register any ports.  We then perform PNPBIOS scanning, which re-registers
ttyS0.  Unfortunately, if devfs is enabled, devfs reports an error because
we try to create two tts/0 entries.

Therefore, when adding a new port we check that it has not been detected
before attempting to probe the port and register it with devfs (via the
tty layer.)

drivers/serial/core.c

@@ -2405,6 +2405,10 @@ int uart_register_port(struct uart_driver *drv, struct uart_port *port)
 			goto out;
 		}
 
+		/*
+		 * If the port is already initialised, don't touch it.
+		 */
+		if (state->port->type == PORT_UNKNOWN) {
 			state->port->iobase   = port->iobase;
 			state->port->membase  = port->membase;
 			state->port->irq      = port->irq;
@@ -2416,6 +2420,7 @@ int uart_register_port(struct uart_driver *drv, struct uart_port *port)
 			state->port->line     = state - drv->state;
 
 			__uart_register_port(drv, state, state->port);
+		}
 
 		ret = state->port->line;
 	} else