ath9k_htc: Really fix device hotunplug

All commands to the target are disabled when the device
is unplugged, but a normal module unload has to be
differentiated from this case, as we could still receive
data in the RX endpoint. Fix this by checking if the
device is attached or not.

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