nvme-pci: split out a nvme_pci_ctrl_is_dead helper

Clean up nvme_dev_disable by splitting the logic to detect if a
controller is dead into a separate helper.

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