iwl3945: fix bugs in txq freeing

When we free a txq that had no txb array allocated,
we still try to access it. Fix that, and also free
all SKBs that may be in the txb array (although it
can just be a single one).

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>