IB/mad: Return EPROTONOSUPPORT when an RDMA device lacks the QP required

We had a script which was looping through the devices returned from
ibstat and attempted to register a SMI agent on an ethernet device.
This caused a kernel panic for IBoE devices that don't have QP0.

Fix this by checking if the QP exists before using it.

Signed-off-by: Ira Weiny <weiny2@llnl.gov>
Acked-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>