mtip32xx: fix device removal

Use the proper helper to mark a surpise removal, remove the gendisk as
soon as possible when removing the device and implement the ->free_disk
callback to ensure the private data is alive as long as the gendisk has
references.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/r/20220619060552.1850436-3-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>