serial: core: add comment about definitely used port types

When port type 18 was removed, it was deduced that the code could go but
its define has to stay because it is used in userspace. Share that
knowledge by adding a comment about it.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20230922063642.4120-3-wsa+renesas@sang-engineering.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>