ath9k_htc: Use SKB's private area for TX parameters

For all packets sent through the USB_WLAN_TX_PIPE endpoint,
the private area of the SKB's tx_info can be used to store
driver-specific information. For packets sent through USB_REG_OUT_PIPE,
this will not make a difference since they are routed through a
separate routine that doesn't access the private region.

This would help in situations where TX information is required
in the URB callback.

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