staging: rtl8712: fix use-after-free in rtl8712_dl_fw

Syzbot reported use-after-free in rtl8712_dl_fw(). The problem was in race condition between r871xu_dev_remove() ->ndo_open() callback.

It's easy to see from crash log, that driver accesses released firmware in ->ndo_open() callback. It may happen, since driver was releasing firmware _before_ unregistering netdev. Fix it by moving unregister_netdev() before cleaning up resources.

Call Trace:
...
 rtl871x_open_fw drivers/staging/rtl8712/hal_init.c:83 [inline]
 rtl8712_dl_fw+0xd95/0xe10 drivers/staging/rtl8712/hal_init.c:170
 rtl8712_hal_init drivers/staging/rtl8712/hal_init.c:330 [inline]
 rtl871x_hal_init+0xae/0x180 drivers/staging/rtl8712/hal_init.c:394
 netdev_open+0xe6/0x6c0 drivers/staging/rtl8712/os_intfs.c:380
 __dev_open+0x2bc/0x4d0 net/core/dev.c:1484

Freed by task 1306:
...
 release_firmware+0x1b/0x30 drivers/base/firmware_loader/main.c:1053
 r871xu_dev_remove+0xcc/0x2c0 drivers/staging/rtl8712/usb_intf.c:599
 usb_unbind_interface+0x1d8/0x8d0 drivers/usb/core/driver.c:458

---
 drivers/staging/rtl8712/usb_intf.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index 505ebeb643dc..cae04272deff 100644
@@ -595,12 +595,12 @@ static void r871xu_dev_remove(struct usb_interface *pusb_intf)
 
 	/* never exit with a firmware callback pending */
 	wait_for_completion(&padapter->rtl8712_fw_ready);
+	if (pnetdev->reg_state != NETREG_UNINITIALIZED)
+		unregister_netdev(pnetdev); /* will call netdev_close() */
 	usb_set_intfdata(pusb_intf, NULL);
 	release_firmware(padapter->fw);
 	if (drvpriv.drv_registered)
 		padapter->surprise_removed = true;
-	if (pnetdev->reg_state != NETREG_UNINITIALIZED)
-		unregister_netdev(pnetdev); /* will call netdev_close() */
 	r8712_flush_rwctrl_works(padapter);
 	r8712_flush_led_works(padapter);
 	udelay(1);

2.33.1