nvme-rdma: fix nvme_rdma_queue_is_ready

Now that we don't abuse the cmd field in struct request for nvme command
passthrough this function needs to be converted to the proper accessor
as well.

Fixes: d49187e9

 ("nvme: introduce struct nvme_request")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>