staging: fieldbus: make controller_class constant

staging: fieldbus: make controller_class constant

Now that the driver core allows for struct class to be in read-only
memory, making all 'class' structures to be declared at build time
placing them into read-only memory, instead of having to be dynamically
allocated at load time.

Cc: Sven Van Asbroeck <TheSven73@gmail.com>
Link: https://lore.kernel.org/r/2023100552-entrench-dingbat-093a@gregkh
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>