can: kvaser_usb: Ignore CMD_FLUSH_QUEUE_REPLY messages

To avoid kernel warning "Unhandled message (68)", ignore the
CMD_FLUSH_QUEUE_REPLY message for now.

As of Leaf v2 firmware version v4.1.844 (2017-02-15), flush tx queue is
synchronous. There is a capability bit indicating whether flushing tx
queue is synchronous or asynchronous.

A proper solution would be to query the device for capabilities. If the
synchronous tx flush capability bit is set, we should wait for
CMD_FLUSH_QUEUE_REPLY message, while flushing the tx queue.

Signed-off-by: Jimmy Assarsson <jimmyassarsson@gmail.com>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>