iwlagn: move Tx datapath to transport layer

Split the Tx datapath in two parts:
* the first deals with the Tx cmd composition
* the second attaches the skb + Tx cmd to the queues

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>