serial: 8250: document doubled "type == PORT_8250_CIR" check

The check for "port.type == PORT_8250_CIR" is present twice in
serial8250_register_8250_port(). The latter was already tried to be
dropped by 1104321 ("serial: Delete dead code for CIR serial
ports") and then reverted by 9527b82 ("Revert "serial: Delete dead
code for CIR serial ports"").

Document this weirdness with a reason.

Signed-off-by: "Jiri Slaby (SUSE)" <[email protected]>
Suggested-by: Andy Shevchenko <[email protected]>
Acked-by: Andy Shevchenko <[email protected]>
Cc: "Maciej S. Szmigiero" <[email protected]>
Link: https://lore.kernel.org/all/[email protected]/
Reviewed-by: Ilpo Järvinen <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Jiri Slaby (SUSE)

drivers/tty/serial/8250/8250_core.c

@@ -717,6 +717,7 @@ int serial8250_register_8250_port(const struct uart_8250_port *up)
 		nr_uarts++;
 	}
 
+	/* Check if it is CIR already. We check this below again, see there why. */
 	if (uart->port.type == PORT_8250_CIR) {
 		ret = -ENODEV;
 		goto unlock;
@@ -815,6 +816,7 @@ int serial8250_register_8250_port(const struct uart_8250_port *up)
 	if (up->dl_write)
 		uart->dl_write = up->dl_write;
 
+	/* Check the type (again)! It might have changed by the port.type assignment above. */
 	if (uart->port.type != PORT_8250_CIR) {
 		if (uart_console_registered(&uart->port))
 			pm_runtime_get_sync(uart->port.dev);