wifi: ath9k: hif_usb: fix memory leak of urbs in ath9k_hif_usb_dealloc_tx_urbs()

In the Linux kernel, the following vulnerability has been resolved:

wifi: ath9k: hif_usb: fix memory leak of urbs in ath9k_hif_usb_dealloc_tx_urbs()

Syzkaller reports a long-known leak of urbs in
ath9k_hif_usb_dealloc_tx_urbs().

The cause of the leak is that usb_get_urb() is called but usb_free_urb()
(or usb_put_urb()) is not called inside usb_kill_urb() as urb->dev or
urb->ep fields have not been initialized and usb_kill_urb() returns
immediately.

The patch removes trying to kill urbs located in hif_dev->tx.tx_buf
because hif_dev->tx.tx_buf is not supposed to contain urbs which are in
pending state (the pending urbs are stored in hif_dev->tx.tx_pending).
The tx.tx_lock is acquired so there should not be any changes in the list.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.