vfio/mdpy: make mdpy_class constant

vfio/mdpy: make mdpy_class constant

Since commit 43a7206b0963 ("driver core: class: make class_register() take
a const *"), the driver core allows for struct class to be in read-only
memory, so move the mdpy_class structure to be declared at build time
placing it into read-only memory, instead of having to be dynamically
allocated at boot time.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Link: https://lore.kernel.org/r/20240301-class_cleanup-vfio-v1-1-9236d69083f5@marliere.net
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>