iwlwifi: make iwl_tx_queue->tfds void*

Instead of having both tfds and tfds39, we can just have a void *tfds.
It makes the tx_queue structure nicer, and the code cleaner. It also helps
with further TX queues management code merging.

Signed-off-by: Samuel Ortiz <samuel.ortiz@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>