nvmet: fix invalid memory reference in nvmet_subsys_attr_qid_max_show

The item passed into nvmet_subsys_attr_qid_max_show is not a member of
struct nvmet_port, it is part of nvmet_subsys.  Hence, don't try to
dereference it as struct nvme_ctrl pointer.

Fixes: 3e980f59

 ("nvmet: Expose max queues to configfs")
Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Link: https://lore.kernel.org/r/20220913064203.133536-1-dwagner@suse.de
Signed-off-by: Daniel Wagner <dwagner@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Acked-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>