iwlagn: add an API to free the TX context

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>