usbnet: don't clear urb->dev in tx_complete

URB unlinking is always racing with its completion and tx_complete
may be called before or during running usb_unlink_urb, so tx_complete
must not clear urb->dev since it will be used in unlink path,
otherwise invalid memory accesses or usb device leak may be caused
inside usb_unlink_urb.

Cc: stable@kernel.org
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>