IB/ehca: Check idr_find() return value

The idr_find() function may fail when trying to get the QP that is
associated with a CQE, e.g. when a QP has been destroyed between the
generation of a CQE and the poll request for it.  In consequence, the
return value of idr_find() must be checked and the CQE must be
discarded when the QP cannot be found.

Signed-off-by: Alexander Schmidt <alexs@linux.vnet.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>