ath9k_htc: Drain packets on station removal

When a station entry is removed, there could still be
pending packets destined for that station in the HIF layer.
Sending these to the target is not necessary, so drain them
in the driver itself.

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