nvme-pci: remove volatile cqes

The completion queue entry is not volatile once the phase is confirmed.
Remove the volatile keywords and check the phase using the appropriate
READ_ONCE() accessor, allowing the compiler to optimize the remaining
completion path.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>