nvme: move queue_count to the nvme_ctrl

nvme: move queue_count to the nvme_ctrl

All all transports use the queue_count in exactly the same, so move it to
the generic struct nvme_ctrl. In the future it will also be maintained by
the core.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-By: James Smart <james.smart@broadcom.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>