scsi: core: Show nr_hw_queues in sysfs

So that we don't use a value of 0 for when Scsi_Host.nr_hw_queues is unset,
use the tag_set->nr_hw_queues (which holds 1 for this case).

Signed-off-by: John Garry <john.garry@huawei.com>
Tested-by: Douglas Gilbert <dgilbert@interlog.com>
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>