[PATCH] IB/mthca: Simplify handling of completions with error

Mem-free HCAs never generate error CQEs that complete multiple WQEs,
so just skip the call to mthca_free_err_wqe() for them rather than
having logic to handle the mem-free case in mthca_free_err_wqe().

Signed-off-by: Roland Dreier <rolandd@cisco.com>