nvme-pci: simplify nvme_cqe_valid

We always look at the current CQ head and phase, so don't pass these
as separate arguments, and rename the function to nvme_cqe_pending.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>