misc: phantom: make phantom_class constant

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

Cc: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/2023102434-font-feast-98e3@gregkh
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>