nvme-pci: set cqe_seen on polled completions

Fixes: 920d13a8 ("nvme-pci: factor out the cqe reading mechanics from __nvme_process_cq")
Reported-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>