nvme-pci: open-code polling logic in nvme_poll

Given that the code is simple enough it seems better
then passing a tag by reference for each call site, also
we can now get rid of __nvme_process_cq.

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