nvmet-rdma: check for timeout in nvme_rdma_wait_for_cm()

Check whether queue->cm_error holds a value before reading it. This patch
addresses Coverity ID 1373774: unchecked return value.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>