nvme: enable aen regardless of the presence of I/O queues

AENs in general are not related to the presence of I/O queues,
so enable them regardless. Note that the only exception is that
discovery controller will not support any of the requested AENs
and nvme_enable_aen will respect that and return, so it is still
safe to enable regardless.

Note it is safe to enable AENs even before the initial namespace
scanning as we have the scan operation in a workqueue context.

Reviewed-by: Minwoo Im <minwoo.im.dev@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>