bcache: Fix register_device_aync typo

Should be `register_device_async`.

Cc: Coly Li <colyli@suse.de>
Signed-off-by: Kai Krakow <kai@kaishome.de>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>