can: ti_hecc: ti_hecc_stop(): stop the CPK on down

When the interface goes down, the CPK should no longer take an active
part in the CAN-bus communication, like sending acks and error frames.
So enable configuration mode in ti_hecc_stop, so the CPK is no longer
active.

When a transceiver switch is present the acks and errors don't make it
to the bus, but disabling the CPK then does prevent oddities, like
ti_hecc_reset() failing, since the CPK can become bus-off and starts
counting the 11 bit recessive bits, which seems to block the reset. It
can also cause invalid interrupts and disrupt the CAN-bus, since
transmission can be stopped in the middle of a message, by disabling the
tranceiver while the CPK is sending.

Since the CPK is disabled after normal power on, it is typically only
seen when the interface is restarted.

Signed-off-by: Jeroen Hofstee <jhofstee@victronenergy.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>