ath9k_htc: Handle device unplug properly

When the USB device has been unplugged, there is
no point in trying to send commands to the target.
Fix this by denying all WMI commands in such a case.

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