nvme: remove handling of multiple AEN requests

The driver can handle tracking only one AEN request, so this patch
removes handling for multiple ones.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: James Smart <james.smart@broadcom.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>