ath9k_htc: fix memory leak in ath9k_hif_usb_alloc_urbs

Failure cases within ath9k_hif_usb_alloc_urbs are failed
to release allocated memory.

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