RDMA/i40iw: Set vendor_err only if there is an actual error

Add a check for cq_poll_info.error before setting vendor_err
instead of always setting it.

Signed-off-by: Mustafa Ismail <mustafa.ismail@intel.com>
Signed-off-by: Faisal Latif <faisal.latif@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>