iwlagn: upper layer stores iwl_rxon_context in skb's CB

This removes the need for iwl_tx_info.
Each tx queue holds an array of skbs, the transport layer doesn't
need to know anything about the context in which a specific skb is
sent.

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>