carl9170: remove set but not used variable 'udev'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/ath/carl9170/usb.c: In function carl9170_usb_disconnect:
drivers/net/wireless/ath/carl9170/usb.c:1110:21:
 warning: variable udev set but not used [-Wunused-but-set-variable]

It is not use since commit feb09b29 ("carl9170:
fix misuse of device driver API")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ath/carl9170/usb.c

@@ -1107,12 +1107,10 @@ static int carl9170_usb_probe(struct usb_interface *intf,
 static void carl9170_usb_disconnect(struct usb_interface *intf)
 {
 	struct ar9170 *ar = usb_get_intfdata(intf);
-	struct usb_device *udev;
 
 	if (WARN_ON(!ar))
 		return;
 
-	udev = ar->udev;
 	wait_for_completion(&ar->fw_load_wait);
 
 	if (IS_INITIALIZED(ar)) {