ath6kl: Fix a possible null-pointer dereference in ath6kl_htc_mbox_create()

In ath6kl_htc_mbox_create(), when kzalloc() on line 2855 fails,
target->dev is assigned to NULL, and ath6kl_htc_mbox_cleanup(target) is
called on line 2885.

In ath6kl_htc_mbox_cleanup(), target->dev is used on line 2895:
    ath6kl_hif_cleanup_scatter(target->dev->ar);

Thus, a null-pointer dereference may occur.

To fix this bug, kfree(target) is called and NULL is returned when
kzalloc() on line 2855 fails.

This bug is found by a static analysis tool STCheck written by us.
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ath/ath6kl/htc_mbox.c

@@ -2855,8 +2855,8 @@ static void *ath6kl_htc_mbox_create(struct ath6kl *ar)
 	target->dev = kzalloc(sizeof(*target->dev), GFP_KERNEL);
 	if (!target->dev) {
 		ath6kl_err("unable to allocate memory\n");
-		status = -ENOMEM;
-		goto err_htc_cleanup;
+		kfree(target);
+		return NULL;
 	}
 
 	spin_lock_init(&target->htc_lock);