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

drivers/net/can/usb/ucan.c: In function 'ucan_disconnect':
drivers/net/can/usb/ucan.c:1578:21: warning:
 variable 'udev' set but not used [-Wunused-but-set-variable]
  struct usb_device *udev;
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Martin Elshuber <martin.elshuber@theobroma-systems.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>