ath9k_htc: Sync MGMT/DATA packet headers with firmware

Add a new cookie field that would be filled by the host.
This can be used to match the TX status WMI event with
the appropriate packet.

Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>