can: c_can: end pending transmission on network stop (ifdown)

Put controller into init mode in network stop to end pending transmissions. The
issue is observed in cases when transmitted frame is not acked.

Signed-off-by: Viktor Babrian <babrian.viktor@renyi.mta.hu>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>