nvme-pci: use a flag for polled queues

A negative value for the cq_vector used to mean the queue is either
disabled or a polled queue. However, we have a queue enabled flag,
so the cq_vector had been serving double duty.

Don't overload the meaning of cq_vector. Use a flag specific to the
polled queues instead.

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