Tx free functions move to the transport layer. Unify the functions that deal with tx queues and cmd queue.

Since the CMD queue is not fully allocated, but uses the q->n_bd / q->window trick, the release flow of TX queue and CMD queue was different.
iwlagn_txq_free_tfd receives now the index of the TFD to be freed, which allows to unify the release flow for all the queues.
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>