Merge branch 'isdn-constify-struct-class-usage'

Ricardo B. Marliere says:

====================
isdn: constify struct class usage

This is a simple and straight forward cleanup series that aims to make the
class structures in isdn constant. This has been possible since 2023 [1].

[1]: https://lore.kernel.org/all/2023040248-customary-release-4aec@gregkh/
====================

Link: https://lore.kernel.org/r/20240305-class_cleanup-isdn-v1-0-6f0edca75b61@marliere.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>