NVMe: Return real error from nvme_create_queue

nvme_setup_io_queues() was assuming that a NULL return from
nvme_create_queue() was an out-of-memory error.  That's not necessarily
true; the adapter might return -EIO, for example.  Change the calling
convention to return an ERR_PTR on failure instead of NULL.

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>