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

Only request an IRQ mapping for read queues if at least one read queue
is being allocted, as nvme_pci_map_queues() will later on ignore the
unnecessary mapping request should nvme_dev_add() request such an IRQ
mapping even though no read queues are being allocated.  However,
nvme_dev_add() can avoid making the request by checking the number of
read queues without assuming. This would bring it more in line with
nvme_setup_irqs() and nvme_calc_irq_sets().

Signed-off-by: Alan Mikhak <alan.mikhak@sifive.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>