Revert "nvme-pci: don't create a read hctx mapping without read queues"

This reverts commit 0298d543

.

With this patch, set 'poll_queues > hard queues' will lead to 'nr_read_queues = 0'
in nvme_calc_irq_sets. Then poll_queues setting can fail since dev->tagset.nr_maps
equals to 2 and nvme_pci_map_queues will not do map for poll queues.

Signed-off-by: yangerkun <yangerkun@huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>