driver core: class: mark the struct class in struct class_interface constant

The struct class pointer in struct class_interface is never modified, so
mark it as const so that no one accidentally tries to modify it in the
future.

Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/2023040249-handball-gruffly-5da7@gregkh


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