can: usb_8dev: Fix memory leak in usb_8dev_start_xmit

Fixed a memory leak when an error occurred in the transmit function. In the
error handling the urb wasn't freed before returning. There was also a call to
the usb_unanchor_urb() function but the urb wasn't anchored.

Signed-off-by: Bjorn Van Tilt <bjorn.vantilt@gmail.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>