ath9k: fix use-after-free in ath9k_hif_usb_rx_cb

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

ath9k: fix use-after-free in ath9k_hif_usb_rx_cb

Syzbot reported use-after-free Read in ath9k_hif_usb_rx_cb() [0]. The

problem was in incorrect htc_handle->drv_priv initialization.

Probable call trace which can trigger use-after-free:

ath9k_htc_probe_device()

/* htc_handle->drv_priv = priv; */

ath9k_htc_wait_for_target() <--- Failed

ieee80211_free_hw() <--- priv pointer is freed

<IRQ>

...

ath9k_hif_usb_rx_cb()

ath9k_hif_usb_rx_stream()

RX_STAT_INC() <--- htc_handle->drv_priv access

In order to not add fancy protection for drv_priv we can move

htc_handle->drv_priv initialization at the end of the

ath9k_htc_probe_device() and add helper macro to make

all *_STAT_* macros NULL safe, since syzbot has reported related NULL

deref in that macros [1]