iwlwifi: don't rely on the wr / rd pointers in DELBA flow

In the same spirit as the previous patch. Eventually this will
allow us to remove the tid_data knowledge from the transport layer.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>