ath9k_htc: Simplify RX URB management

This patch introduces the usage of URB anchors,
thus reducing a large amount of code dealing with
URB maintenance within the driver. The RX callback now
takes care of freeing the SKB associated with each URB.

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