nvme: store a struct device pointer in struct nvme_dev

Most users want the generic device, so store that in struct nvme_dev
instead of the pci_dev.  This also happens to be a nice step towards
making some code reusable for non-PCI transports.

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