nvme: Add a wrapper for getting the admin queue depth

nvme: Add a wrapper for getting the admin queue depth

The NVMe protocol provides no means for a device to report its maximum
queue depth. To facilitate being able to override the default on a
per-device basis, store the admin queue depth in struct nvme_dev and use
that value when allocating the queue.

Also provide an accessor function for use in place of the
NVME_AQ_BLKMQ_DEPTH macro that will return the right queue depth based
on the value stored in the nvme_dev struct.

Orabug: 26284591

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Ashok Vairavan <ashok.vairavan@oracle.com>