staging: greybus: fw-management: make fw_mgmt_class constant

staging: greybus: fw-management: make fw_mgmt_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: Alex Elder <elder@kernel.org>
Cc: greybus-dev@lists.linaro.org
Reviewed-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/2023100534-catty-moodiness-099e@gregkh
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>