iwlwifi: add iwl_cmd_queue_free for readability

This patch adds iwl_cmd_queue_free function and
separate cmd queue freeing from regular tx queue freeing.
This improves readability and adds one more step in
restructuring  of cmd queue handling

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Reviewed-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>