ath9k_htc: Fix RX URB reference count

RX URBs are automatically freed when the reference
count drops to zero - this currently doesn't happen when
usb_kill_anchored_urbs() is called during unload.

Fix this by dropping the reference count by one during
initial submission.

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