staging/rdma/hfi1: Fix xmit discard error weight

Count only the errors that apply to xmit discards.  Update
the comment to better explain the limitations of the count.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>