can: esd_usb2: fix memory leak on disconnect

It seems struct esd_usb2 dev is not deallocated on disconnect. The patch adds
the missing deallocation.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Acked-by: Matthias Fuchs <matthias.fuchs@esd.eu>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/usb/esd_usb2.c

@@ -1143,6 +1143,7 @@ static void esd_usb2_disconnect(struct usb_interface *intf)
 			}
 		}
 		unlink_all_urbs(dev);
+		kfree(dev);
 	}
 }