mux: constify mux class

All class functions used here take a const pointer to the class
structure so we can make the struct itself constant.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Link: https://lore.kernel.org/r/20241205085605.9501-1-brgl@bgdev.pl


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