qedr: clear the vendor error field in the work completion

We clear the vendor error field in the work completion so that if
a work completion is erroneous  the field won't confuse the caller.

Signed-off-by: Ram Amrani <Ram.Amrani@cavium.com>
Reviewed-by: Michal Kalderon <Michal.Kalderon@cavium.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>