iwlwifi: counting number of tfds can be free for 4965

Forget one hunk in 4965 during "iwlwifi: error checking for number of tfds
in queue" patch.

Reported-by: Shanyu Zhao <shanyu.zhao@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
CC: stable@kernel.org