nvme: merge nvme_dev_start, nvme_dev_resume and nvme_async_probe

And give the resulting function a sensible name.  This keeps all the
error handling in a single place and will allow for further improvements
to it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>