can: slcan: send the open/close commands to the adapter

In case the bitrate has been set via ip tool, this patch changes the
driver to send the open ("O\r") and close ("C\r) commands to the
adapter.

Link: https://lore.kernel.org/all/20220628163137.413025-9-dario.binacchi@amarulasolutions.com
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Tested-by: Jeroen Hofstee <jhofstee@victronenergy.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>