wifi: ath9k: hif_usb: Fix use-after-free in ath9k_hif_usb_reg_in_cb()

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

wifi: ath9k: hif_usb: Fix use-after-free in ath9k_hif_usb_reg_in_cb()

It is possible that skb is freed in ath9k_htc_rx_msg(), then

usb_submit_urb() fails and we try to free skb again. It causes

use-after-free bug. Moreover, if alloc_skb() fails, urb->context becomes

NULL but rx_buf is not freed and there can be a memory leak.

The patch removes unnecessary nskb and makes skb processing more clear: it

is supposed that ath9k_htc_rx_msg() either frees old skb or passes its

managing to another callback function.

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