ath9k_htc: Add TX slots

Maintain a bitmap of slots for transmission and update
the cookie field for every packet with the slot value.
This value would be used for matching packets when TX
completion processing is added.

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