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

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 1104321a7b3b ("serial: Delete dead code for CIR serial
ports") and then reverted by 9527b82ae3af ("Revert "serial: Delete dead
code for CIR serial ports"").

Document this weirdness with a reason.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: "Maciej S. Szmigiero" <mail@maciej.szmigiero.name>
Link: https://lore.kernel.org/all/aFcDOx1bdB34I5hS@surfacebook.localdomain/
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20250624080641.509959-5-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>